That'll between the person who wants in and the person running the thread. Certain points of the rp it's easier to let someone in, other points it's pretty hard. There may be certain points where the person has to wait before they can inter the rp.
Number of posts per day are good where there are either a lot of folks in the rp where some could post a page between only 2-3 players only, or where there may be some who really want to rp in the thread but don't have a lot of time to post per day. But I'm sure some folks already brought that point up, good reminding some though case any may forget and the other members interested in this can put their opinions here.
I also like the idea of having a type of rp thread sometimes that others who usually don't rp, may have a chance to try to rp. This may attract some who may never, or seldom, have rped before.