how much storage do I need

I've run out of space

You haven't stated this, but presumably your limited space is on iCloud rather than your device?

As to how much space you need only you can assess that.

The three iCloud+ plans and their pricing can be found in the Apple support document iCloud+ plans and pricing including the local cost in all the territories iCloud+ is available. Read the document and assess which of the three would suit you best.

If you are moving to iCloud+ subscription from the free 5GB plan it might be worthwhile to start with the 200GB plan, rather than the smaller 50GB plan, but again, this depends on your assessment of how much data and content you have which needs accommodating in iCloud.