If i buy 0.99 euro per month 50 gb , is it regularly i have to pay every month for that 50 gb ???

iCloud storage is a month to month fee. It must be paid every month that you want to use it.

Yes, iCloud storage is available as a subscription. You pay every month for as long as you wish to have and use the storage.

You pay the fee every month and you get a total of 50GB of storage that never increases. It will always be 50GB.

I make the comparison to renting a storage locker. You rent a locker that has a fixed amount of space. The space never gets bigger, but you pay a monthly fee to rent or use that fixed amount of apace and you pay the fee, each and every month until you cancel the storage and move your belongings out of the space.