Well, if you're throwing a party and not having holiday dinner the same time as everyone else, you can borrow from friends that live close to you.

Otherwise, I have found that hitting out of season sales at Home Depot and Lowe's, as well as beginning-of-summer yard sales, to get a chair here and there is the best way. How many extra chairs can one really need? If you have a big house and are having 20 people over for dinner or something, then you very likely do have a large dining room with a long table and plenty of permanent chairs. If you have a smaller place, don't invite too many more folks than you have chairs because you probably don't have the space for them anyway.

Or embrace allowing people to break into smaller groups in the kitchen, dining room, and living room to eat and enjoy a time of smaller, quieter conversations. When dessert comes, people can get up and change groups or wander around easily with only a small dessert plate to handle.

the JEFF at IKEA is amazingly comfy, lightweight and $8.95.
http://www.ikea.com/us/en/catalog/products/30070402/

there are aways options. That's why benches and ottomans are so handy to have in the house.

I only have four chairs so I come across this problem all the time. I always ask guests to bring one or two chairs with them. I'm laying on the food and drink so I think they feel like they're contributing. I don't have room to store extra chairs, if I did I would.

I use fold up chairs from our balcony. There is no room for extra chairs in my place, so my partner and I each usually take one of the balcony chairs and give the guests the more comfy ones.

We keep folded chairs and a cheap folding card table in a closet - there's a slight height difference between the card table and our long formal dining table, but if you throw a table cloth over the whole thing it really doesn't make a giant difference.

My dining room is small, and the table has four Parson's chairs. My partner and I both have the same model desk chair (a Pier One side chair called "Dorothy" which I think is no longer available.) Whe we use the leaf of the table, we use those two chairs for the extra seats (usually he and I sit on them leaving the prettier chairs for our guests.)

For more than 6 we don't do sit-down dinners, just buffets. (And neither one of us have family to invite for Thanksgiving, so big family meals aren't an issue.)

Generally we pull out the lawn chairs from the front porch. Wipe them down and voila! They may be a little different height-wise, but they work fairly well. Our Thanksgiving meal is in the evening so it works out considering the night isn't planned around the table.

Our piano bench is routinely called into service - it seats 2 children nicely at the head of the table.

We have always had a stack of chairs in one of our closets, but recently we renovated our kitchen, breaking through to our dining room. Now we have the Boulangerie table from Restoration Hardware running down the middle of the room. It goes from 8 to 10 feet and we'll have almost all of our chairs there all the time with maybe 2 spare in the closet. This will be a big improvement because the stacking chairs aren't as comfortable and now we will only occasionally need to use two, and my husband and I can take them.

Buffet style is the best.

We have an eight-person table and chairs set, but we usually keep it set with just four chairs since that's a typical amount of total people. (I just live with my husband.) The other four chairs serve as side/end/accent tables, nightstands, suitcase racks – whatever we need them to be – so they're easy to relocate for a bigger gathering.

I upholstered a couple of wooden folding chairs from IKEA and they've been endlessly useful. They're actually more comfortable than my vintage tubular kitchen chairs anyway, so they're rarely folded and stored since my butt usually finds its way on to one :)

I used to have 4 folding chairs stored behind the kitchen door (pic here: http://ploefff.files.wordpress.com/2010/10/stolebagdc3b8ren.jpg) A "lost" space that wouldn't otherwise have been used so 2 in one hit for me. I made some changes to my apartment since and now only have tables that are the same height as my kitchen counter so all chairs can be used where ever they are needed.

Ploefff, that is so smart! I may do this with the back of my laundry room door.

We use a combo of piano bench, outdoor furniture & our office chairs. It helps that the office chairs are the same as the dining room chairs. We have more than enough chairs, just not a room large enough inside to sit around a huge table. Too bad it's not warm enough to sit outside for Thanksgiving, but Boston at the end of November isn't warm enough for me, even with a heater.

Luckily we have 3 family member apartments in one house so between us all there's enough chairs for the gang and extras. My son's place has a vintage 60's teak table with several leaves that expands to hold 12 regular or 16 skinnies, and a set of 6 chairs so my mom's and my chairs get pressed into service. We also have a folding table that the food usually gets put on so we can really cram a crowd in at the main table, The kids get their own table and kid chairs, though the 9 year old is getting fed up with that. We try to arrange the chairs so there's some kind of decent aesthetic effect but the styles are such a crazy mix.

We use the IKEA Jeff for outdoor dining chairs bought 8 at $8 each. We have four chairs for our dining table, 4 chairs for our kitchen table and this Thanksgiving, we're bringing the 8 chairs from outside to sit 16 at our feast!

my apartment is much too small for a sit-down dinner with more than 4 people, but buffets are great. 4 dinning chairs, plus a few bentwood thonets and a couch serve as plenty of seating with tv trays with tablecloths as side tables.

my aunt has the large dining room where we have christmas. her dining room has antique music back chairs that we supplement with folding chairs like the ones below (she found her's on craigslist). she actually just tucks them behind drapes. and there are always piano benches.

http://www.target.com/p/music-back-folding-chair-2pk-cherry/-/A-562739?ref=tgt_adv_XSG10001&AFID=Google_PLA_df&LNM=|562739&CPNG=Furniture&kpid=562739&ci_gpa=pla&ci_sku=562739

I've collected wooden chairs from garage sales and painted them all the same color with matching cushions. I also have ottomans that I use. To store the extra chairs I've used a simple bike rack hook and hang them on the wall to hold a lamp, plant, picture or even books in my kitchen entry

We live in a relatively small house (approx 1000 sq ft) and have found the Flanders chair from Industry West to be great for extra seating. They're extremely sturdy and stack up to 10 high. We keep 4 chairs around the table and keep another 4 stacked up in the corner of our kitchen under our phone where they get used as a sort of impromptu desk for lists/pens/bills.

http://www.industrywest.com/shop/chairs/flanders-chair.html

I have chairs in the bedrooms that are dining chairs so if I need to I can bring them down. I also have an Ikea frosta stool (Eur11) that I usually use as a side table, with a coloured placemat on top of it . If I need to I just whisk off whatever is on it and it becomes a stool again (actually I'm always a little surprised to see it as a stool, I'm so used to seeing it as a side table). And I have two folding chairs as well.

I've been known to slide my dining table over to my couch for 3-4 extra seats, and then rearrange the rest of the chairs around the rest of the table.

ghost chairs are stackable and comfortable and fit in around the table easily.