Other than the HF staff, probably no one knows for sure... I am also interested.
I have a Pro subscription and so far have never incurred any additional fees.
I don't think there is any sort of limit on the total number of times, but there are often cases where a lot of requests in a short period of time will result in a limit (or Quota).

Pro subscription subscribers have different positions at different times, but in your case I think we are talking about APIs.

1. As a Space Publisher
2. AS a Space User
3. As an API User

In your case, I think I might be caught in limit by short period requesting, but I don't know for sure.
I use only Spaces usually (often use APIs in the space), so my information maybe inaccurate.

Incidentally, as a Spaces publisher, $10/month is too cheap, but as a mere Space user of generative AI, the benefit is so far almost nonexistent. Quota is tight. I'm not saying we should be like PixAI, but I wonder if we can do something more...
I don't dislike the free-user-friendly way of doing things, and I think it's one of the most important thing for the development community. It's a little bit...how can I say it...it's not mundane, it's not worldly.

Spec of Zero GPU Space

https://discuss.huggingface.co/t/does-a-pro-subscription-add-memory-to-hf-spaces/103927/3