Not making excuses, but it is summer. School was out, now it will soon start again. Many families are busy with vacations, even couples with no children. Now that school is preparing to begin, many parents are trying to get children ready for school, and get as much summer time fun that may be left. I imagine your guests have placed your RSVP to the side with intentions of replying, and have simply forgotten.
Is there any way you and your bridal party could split up the list and start calling these people? I know that your stress level is off the charts right now trying to tie up loose ends, but if you delegate this duty perhaps you would be able to get a more firm count for your reception.
edit: by the way, every party I have ever planned over the past 25 years in which I requested an RSVP, only 2 people responded. Yes, you are seeing correctly -- 2. Yet most of those invited show up. Now, THAT's rude. And when I mention it, they always remark that they thought it was just for regrets. Received an invitation with RSVP to my niece's wedding next month, and am curious to see how many she receives ( I sent ours immediately). I imagine she will be on the phone a few weeks from now, getting a firm count for her caterers.