The uploaded contacts limit is fixed. Growth plan has a limit of 1000 uploaded contacts and Hyper Growth plan 25 000 contacts. These contacts wouldn't replenish every month.
In case you want to create a room for the new leads, there are a few options:
1. Opt for add-ons.
2. Delete campaigns that you no longer need.
3. Delete the old/irrelevant leads from the campaigns you need.
This article may help if you need an add-on:
How to delete leads in bulk:
⚠️Have in mind that deleted data cannot be recovered.