You don't RSVP???

Almost no one RSVP'd to our engagement party at first. On the event page I clearly said "We do need a specific count of attendees, so if you don't RSVP unfortunately there won't be room for you." Just don't budge on that.

I don't know when your RSVP date vs the date you need to inform your caterer/venue of final head count is but I say a couple days after the RSVP date (maybe like 5 days because snail mail) start the follow up calls with the people who didn't rsvp. Tell them/leave a message saying "if we don't hear back within the next 48 hours we'll assume you regretfully decline."

Oh man. I would be panicked too!! my future family is so big in comparison to mine, my mom was overwhelmed with the amount of invites

I second what Becky and Anna Maria said.

My family is the same way and my father has been assigned to call everyone who hasn't rsvped and get a clear number. We will gave extra just in case, but if you have time to pack a bag and jump in your cousin's car and drive to Maryland, you have time to rsvp and I won't feel bad when you don't have a plate or a chair (or I make you wait to sit down).

My wedding planner actually told me to blame it on her!! Has totally lifted a weight off, as I tell people "you better RSVP or my wedding planner will cut you out, she means business"... people are confirming to everything

The concept of a plated dinner wedding is a bit foreign to my relatives. Especially since kids were also invited I opted for a buffet style to make things easier. But I still needed a headcount. That guest could've just been joking about the whole showing up thing. Once you get the rsvp process going, it may just turn out alright. I recruited my mom to take care of the rsvp for the family invites so that was a huge stress relief. They turned out to be much better about sending them back almost immediately whereas 75% of friends and coworkers were huge procrastinators. Maybe enlist you MIL to help gather the RSVPs and explain that it won't be a "show up" kinda wedding.

You call them and make them understand, in the nicest way possible, that if they're coming, you need a count.

If they 'just show up"? No food and no seat.

You may end up making a lot of calls. You could try talking to your FMIL to see if she can explain to the relatives why they need to actually RSVP. Blame it on the caterer.