Credits from your subscription plan do not accumulate over months, i.e. they are carried over to the monthly ceiling when they expire.

Let's look at an example. If you subscribe on 1 September to the annual Pro plan you have 1000 credits per month. If at the end of September you have 15 unused credits left, on 1 October the number of credits in your account will be reset to 1000 and not 1015.