Hello,
I was wondering recently, would it be possible with the e621 API, to see how many posts there are with X tag?
Would it also be possible to see how many pages X post search query would have?
I can clarify if needed.

The API does not provide this information. Not even an estimate.
Sorry. I know that it would be nice to have.

kora_viridian said: ...

Oh, sorry. I thought OP meant to count posts in a search, not with just one tag. You are right.
The post count at the tags endpoint should be reasonably accurate and up to date, but you cannot assume that it's 100% correct.