It's one per month and you get your first one immediately. So if you signed up today, you will get your 2nd credit August 16th and your 3rd September 16th. Soon as you get your last credit and have redeemed it, you should cancel unless you plan on keeping the subscription @ $14.95/month.
Also don't cancel with credits still on the account because you apparently lose them immediately when you cancel.
Any books you bought with credits are yours to keep so whether you cancel or not, you own them.