I bought storage for my IPhone. Do I have to buy separate storage for my iPad?

If you have subscribed to a higher iCloud storage tier (5GB is free - 50GB, 200GB and 2TB tiers are a subscription service), your iCloud storage is shared between all devices that are signed-in to iCloud with the same AppleID.

As such, all you devices share the same iCloud storage.

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

iCloud storage plans and pricing – Apple Support

No, unless, at some point in the future, you feel you need more iCloud storage for your other iOS devices.