I have an iPhone 13 Pro and have maxed out my iCloud+ of 2 TB
Not sure how you managed that with just an iPhone? Do you record loads of very long videos every day and never delete (offload) anything? 2 TB cloud storage seems like a lot for one person. I always thought that tier was geared towards Family groups (i.e. multiple users).
Check your iCloud storage usage on any device - Apple Support
Verify that you’re not confusing the iPhone’s own disk storage. iPhone can be full regardless of available iCloud space.
I read that I could get Apple One which would add another 2 TB of storage..
It is a bit more nuanced than that. Only Apple One Premier has 2 TB. Apple One Individual and Apple One Family have less (50 GB and 200 GB respectively). Normally for equal storage it means that the singular service gets canceled, but if you need more, then you can add both, for a maximum of 4 TB.
What happens to your iCloud storage when you sign up for Apple One - Apple Support
How do I find out if I already have Apple One.
You only have that if you have chosen to subscribe to that. No one has it without knowing.
See your subscriptions overview - Apple