how muxh data do you need in order to back up your phone?

Again it depends upon how much data you have, and your entire iCloud demand. If you use iCloud to store photos you can use up a fair bit of that 50 for photos alone. My phone can hold 64 GB but if I used up the entire 64 I would definitely need more than the 5 I have. As it is, I hardly keep anything on my iPhone so I don't use much at all. Now if you subscribe to Apple Music and have 50 GB of music, that doesn't count against your 50 GB.

As you can see it gets complicated quickly but it really depends upon how much you have and what kind and if you consider it worth backing up. Some of what I have on my phone are maps I have downloaded for offline use, but I don't bother backing those up because I can re-download them from the app developer should I need to.

It depends upon how much you have. I have 2 iPads and 3 iPhones backed up to my account and it takes up 1.2 GB of the 5 free GB Apple gives me. That said, I don't have much on my devices. Maybe a dozen photographs.