If I purchase plan of 1 month that is 50GB , so it’s mandatory to utilised that50GB in month ?

iCloud is not a consumable. Its storage space. You pay for 50GB, you get 50GB, whether you have files stored there that use it all or not.

The next month, you need to pay again if you want to continue access those 50GBs. If you stop paying it reverts to the free 5GB option.

Its a rental of storage space, not a purchase of consumable data.

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More to the point of your post, when you purchase the iCloud Storage of 50gb, that is the space you are allotted in iCloud for storage, you don't HAVE to use the entire thing, but that's what you get. It's not like your cellular provider where you are given a set amount you can use and then bank either, its 50gb through the subscription length, however long you use it.