21 days out, no idea who’s coming to the wedding

I know what you mean about people not knowing etiquette! We had to literally hunt people down for answers! I have an app downloaded so that if I got a verbal confirmation I could quickly enter it to keep track

You probably don't need to be told this but assuming guests are coming is very dangerous. I would call everyone who hasn't responded and tell them if you don't hear back from them by a certain day and time that you're assuming they will not be able to make it and their seat won't be reserved.

If you tell the caterer a higher head count and people don't show up then you're going to pay more money for food that you didn't need. I'd highly suggest reaching out to those that need it and be pretty blunt on needing an answer as soon as possible. I'd also suggest talking with your caterer on last minute changes. My caterer has a head count but told me that I could add and subtract from that number a week in advance because he doesn't order the food until later anyways. They always want a general number for seating and planning but they can make minor changes somewhat last minute.