If there is only a single setup, you can monitor these stats yourself perfectly of course. I don't think the ACME protocol has such features I'm afraid. You can check for yourself in RFC 8555.
By "If there is only a single setup", do you mean to ask "if the certificates are only managed from a single place in the code"?
They currently are, but as we grow we expect to possibly have multiple build pipelines and other processes managing different certificates under the same account (since, as far as I understand, Let's Encrypt's server counts limits per registered domain). I could probably create my own logging and tracking database but I'd like to avoid such a large task if possible (no enough resources).
EDIT: Just and update - a quick look at RFC 8555 seems to support your thinking that the protocol doesn't support such an action.
Next question is - does Let's Encrypt provide extra API (or any interface, even a GUI portal) on top of the RFC or do they strictly stick with the RFC 8555 protocol?
We do not have an API call by which you can check your account usage. Doing something like using a proxy or setting up monitoring server-side is the best way to accomplish this.
If at some point we do decide to implement something by which you can check your usage/where you are in relation to a Let's Encrypt rate limit, we will likely post it both here on the forum and on our upcoming features page: Upcoming Features - Let's Encrypt - Free SSL/TLS Certificates