How much icloud backup storage capacity do i need?

That would likely be more than adequate but you say nothing about your equipment or usage. I am very non-typical in that I find 5 GB is enough for me with 3 iPhones and 2 iPads backed up to iCloud and still several GB remaining. It all depends upon how you use iCloud and your devices. If somebody had, say, a 128 GB iPhone, I would probably suggest they have enough to accommodate 128 GB at least. If you have multiple devices then don't forget that the content on some will be duplicated. If you have 10 GB of photos on one phone and iCloud is mirroring that content to your iPad, you don't need 20 GB for those photos; they only take 10 GB iCloud storage.