Do you set up plates and seating for "surprise guests" who did not rsvp but show up?

I would definitely call those who haven't RSVPed and let them know that if you do not hear from them by a specific date, then they are considered as a "Won't attend".
In terms of accommodating, 1-2 people is probably the max that would be possible. If a vendor asks for exact numbers, there's a reason for it.

Oh, that's not cute! I didn't make any arrangements for people who did not tell me they were coming. If they didn't RSVP by the deadline I gave them a call to check, but if they said no at that time then I did not make any plans to accommodate them. If someone isn't giving you a clear answer I would tell them that you're sorry you're unable to be more flexible but the venue requires a final head count in order to provide a seat and a meal for them and if they do not give you their answer they will not be able to attend. It's well within your rights as host to make this very reasonable request.

Yeah if you have a deadline then people should respect that. weddings can already be pricey enough and if people show up and don't rsvp by the deadline, I would say too bad so sad.

Oh my gosh, this is so frustrating. I am not looking forward to this at all!

I would honestly not set any space for "surprise" guests. You gave them a deadline and you have to pay per plate. they should respect that!

I would tell those people who are thinking of "surprising" you that you need a hard answer by a certain date. Anything after that is deemed as cancelled and there will not be space for them. I agree that it is unfair and I don't think you should have to pay the extra on the off chance that people may show up. I think you need to explain to everyone the cost of them neither confirming nor denying.