Another b'day party RSVP question

If we gave 4 weeks notice, would it be ok to either ask for RSVPs back in two weeks PRIOR to the party? I'd like some wiggle room so if we get a lot of declines that we can invite some other people. I just have no idea with her class if most would come or not.

I'm not saying I have an A and B list but we are inviting DD's entire class so I am close to my limit to begin with. There are a few extra folks that I would invite if we were much below 20. And if we are below, I wouldn't be as concerned with parents that wanted to bring siblings.

Thoughts? PS We've been invited to a bunch of parties and for the life of me can't remember when the RSVP was due. And most people gave 2-3 weeks notice, not 4 so maybe I'm sending them too early? I know the other thread said 3-4 was fine but I'm wondering if 3 is "better" than 4.

If I did 3 weeks notice, is asking for the RSVP in 1 week to soon? TIA!

With 4 weeks notice I think an RSVP in 2 weeks is ok. That said, my guess is you might not get many.

It seems to me like most party's ask for RSVP 1 week prior to the event.