I would ask. Some of my vegan friends do like fake meat; some don’t. Or some like bean burgers (and I do as well.) It’s all good. Normally, at every cookout or meal at my house I have, I have non-meat options available, like main course options not just some afterthought, as cooking vegan is a fun challenge.
Impossible Burgers are indistinguishable from real (albeit mediocre-quality) meat… if they’re cooked exactly right. But if they’re not cooked exactly right, they don’t taste like real meat that hasn’t been cooked exactly right; they just taste absolutely bizarre.
The real key is the meat substitutes that don’t try to taste like meat, but instead just try to taste good. Some of those succeed quite well, to the point that I might even choose one of them over a real burger, depending on what I’m in the mood for.
I don’t even see “leftover condiments” as a thing. I mean, I have mustard and relish and so on in my fridge all the time. I probably wouldn’t even need to buy any at all in preparation for a party, and if I did, it’d be because what I have in the fridge is running low, and so the new jar would just replace the old one.
I would not take a poll. I wouldn’t want my guests to feel they must remain committed - in either direction - to the response they gave.
Also, why do some of you view being stuck with leftovers as a bad outcome? Make more than you know you will need; nobody will go hungry and, bonus, leftovers!
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We have a lot of get-togethers in the summer that involve burgers and hot dogs. Our headcount is around 15. I usually do 1 per person of each plus maybe 5 extra of each. I buy enough buns for what we’re cooking. Kids only have one of whatever they’re eating and there aren’t too many guests that double up on one of the meat choices. Usually people have one hot dog and one burger. We have so many sides that everyone is pretty filled up. Plus there’s dessert! I always have hot dogs in the freezer just in case.
I like leftover hotdogs, hamburgers not so much. My husband will eat both leftover.
In my experience, a good rule of thumb is 400 grams (roughly 1 pound) of meat per person.
Have a recipe you can share or link to? Sometimes I have a hard time using up buns before they go bad.
I just use a very basic one, this one:
Thank you
From what I’ve read, you should probably not freeze hot dogs for more than about two months. Some time ago I took out a pack of hot dogs that had been frozen for well over a year, and they looked and smelled so odd when I tried to grill them that I threw them out. OTOH, I grilled a marinated Cornish game hen that had been vacuum packed and frozen for more than two years, and it was fine. Be careful with freezing hot dogs.
I guess a lot depends on the size of both, especially the burgers. The pre-made fresh burger patties I always buy vary between at least 1/3 of a pound to about half a pound. Served on jumbo buns with cheese and toppings, they’re usually a meal in itself, with or without extras like fries or potato salad.
But sizes vary and appetites vary, I guess. For large parties I’d certainly make sure I had extras, but with large burgers and jumbo hot dogs, I’d say one each per person plus 20% extra is excessive. I could hardly imagine anyone having one half-pound burger and fries wanting anything else! But of course if there are dogs around (the four-legged kind) all bets are off – cook everything you have! 
I’d be more than tempted. Right around the time I was graduating high school, I was briefly (maybe a year and a half) a vegetarian for fashion reasons. It was a plate full of grilled bratwursts at a backyard cookout that brought the whole thing down.
I don’t really care for grilled hot dogs (I prefer 'em steamed) but other sausages can be good: Italian, Mexican chorizo, Filipino Longanisa. I’ve also been known to grill finger-sized breakfast links over charcoal which gives them a bit of smoke that’s normal for bacon but also delicious on sage-forward links.
Every grocery store in Boston has Italian sausages, but I have trouble finding good German ones. I still remember a little storefront sausage place I went to in Munich; damn, it was good.