If your mailing list has grown since you purchased a Moosend Monthly Plan that can only mean you're doing great work. Congratulations!
At this rate there will come a point when you'll try to send your email campaign out but the platform will inform you that the mailing list you're trying to reach exceeds the size of your current Monthly Plan.
When this happens all you have to do is move ahead with a different Monthly Plan, one that corresponds to a larger list than the one you currently own so that it can accommodate it. The best thing to do is to aim for a Monthly Plan with the appropriate size, bearing in mind how your Mailing List will be growing during the month of the purchase.