Kindly I need to buy storage 20 to 50 GB only one time ,not as monthly subscription

I need to buy storage 20 to 50 GB only one time ,not as monthly subscription

If you are talking about iCloud storage:

iCloud storage 20 GB does not exist.

No, it is not possible to upgrade only for 1 month / 1 time.

Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

You pay monthly for iCloud storage.

The subscription runs as long as you don't downgrade.

You cannot deactivate/pause the subscription for 1 month, but you can downgrade to free (5 GB).

The iCloud + subscription will be terminated depending on the start date.

If you start on August 1 and are going to cancel now, the subscription will stop on November 1, a monthly term is used.

See this Apple article: Downgrade or cancel your iCloud+ plan

Downgrade or cancel your iCloud+ plan - Apple Support (PH)

You'll need to subscribe, then cancel when you wish.