Most serving teams have 18 guests in their station (usually divided into 3 or 4 tables). I imagine in your case as long as all your reservations are linked, they would have all 21 with the same servers. How they arrange the tables you may be able to influence (either put in a request in advance or see how it is set up on night 1 and talk to your serving team if you want it changed for the next nights). I would recommend no more than 8 at a table - we find beyond that, the tables get crowded and it becomes difficult for everyone to be interacting anyway. (Plus if you push too many tables together, it can block pathways to and around other tables. We once were next to a table that was pushed together to seat 16, the server from that station was constantly hitting me as he tried to walk around their table because the pathway that would normally be between the two tables wasn't there anymore.)