Not only are people not responding but others who do RSVP are being rude by crossing out plus 1 and add plus 2 or plus 3! What? Stick to your guns and call the individuals and tell them this is a private reception not a free party and they cannot invite all your friends. As to the ones who do not RSVP, if you do get a response too close to the time and you cannot accommodate them, tell them you are sorry but no RSVP no sttendance. It is not your fault you have provided everything they needed to return the RSVP.
What happens sometimes is: Many of them still show up at the wedding anyway expecting a seat. Now your dilemma is do you let them in or turn them away at the door?
I cannot tell you what to do, it is going to be up to individual brides to make a decision based on the situation. if these are relatives, you will have to live with them long after the wedding so deal with it as best as possible. Many people get someone to call invitees to see if they are attending the wedding.