Every premium video added to a project that is exported is counted towards your monthly quota. As soon as you export a video, your premium video count will be deducted by 1.
If you use the same video footage multiple times in a project, it will be counted as 1 credit. If you use the same video footage in 2 different projects, it will be counted as 2 credits.
Your premium video count will be reset on the day when you have made the payment. So, if you bought the premium videos on the 16th, then your premium videos count will reset on the 16th of every month. Any premium videos remaining at the end of the month will not be carried forward to the next month's quota.