An upgraded iCloud storage plan does renew automatically each month. You don't get more storage each month but do automatically pay for the 50GB for that month (sort of like renting physical storage space or an apartment, the space doesn't get larger each month).
You can use the following instructions to see how much storage you have used and how much is available: Manage your iCloud storage - Apple Support
If you have used most of the 50GB plan and want to upgrade to a larger plan, see: Upgrade to iCloud+ - Apple Support
and
iCloud+ plans and pricing - Apple Support
Note that you don't have to anything to your current plan if you decide to upgrade, it is just upgraded to a larger plan.