repost from BHB, invite question

for girls who did an a and b list, did you just order a certain amount of invites with an earlier RSVP date so you could send them early and as they decline, invite other people?
Right now my wedding is July 21st, rsvp date is June 21st, and I planned on sending out the invites around May 14th.
If I wanted to order a certain amount of invites with an earlier RSVP date, how much earlier should I do it? TIA