I dealt with something very similar, I think a lot of us do. However, the bottom line is that this is you and your SO's wedding. Parent's get excited, sometimes so excited they forget common sense and your feelings.

My husband and I gave our parents an allotted number of people they could invite; my MIL thought that meant number of couples...so we had to ask her to cut it down. She again came back with more than her number, and we had to explain to them that each person costs money, and we didn't want a large expensive wedding for strangers.

I was young and inexperienced w/ my MIL; she ended up inviting more people than I did; I ended up cutting friends from my list because I was so worried about costs and not going over our limit (think food, plates, chairs, you have to rent all that shit.)

Hindsight, I regret it; I should have taken a much firmer stance on how many people were invited and made it more about my husband and our friends. We had people she invited that seriously showed up, watched the ceremony, ate the food, and left immediately after. What a waste of money and stress.

MIL is also heavily involved in a church, and hosted an open invitation shower and then informed me that we needed to invite any guests who brought a gift because that was polite. At that point my SO and I both said hell no, that's rude of her to ask and assume, and it was her idea to have an open shower, so she can kindly tell them they're not attending our wedding.

Bottom line is this; you tell your parents they get XXX amount of guests, if those guests RSVP no, then too bad. Once the guest list is finalized, it's finalized. NO REVISIONS. It's too much stress and time to have to continually update that, change things, send out last minute invites, etc. This is your wedding, not theirs. You're paying for it, so really you do have more control.