You can't get your final numbers and pay your suppliers until your RSVP's are back. So your in your guests hands!
So send your invites earlier than you need to, give people more time. Put your RSVP date earlier than you need as well - this will give your guests a little longer to reply, and you a little longer if they forget to reply. Lets face it, you know you are going to be chasing up at least one couple!