I am not clear if you are asking how many in terms of product quantity or how many individual characters in the line? The latter would be easy to find out by counting up on most ID sites, for G1 and for G3. Also G4 if you used MLPMerch.
I don't see the point in wanting to know about individual ponies sold in the US, though, since both the US line and the 1992 cutoff date are arbitrary and irrelevant in the big picture of G1. I am not sure if that is true for G3, may depend if you want to include 3.5 or core 7.
BUT if you are talking about the number of products overall, yeah, it would have to be Hasbro. And I will guarantee that they almost certainly don't know.
With the animation, the people who would know, aside Sunbow as a company, would be those who were involved in the writing, production, storyboard, script etc. I've said it before but we're lacking scripts and other material for MLP. With Jem there's a lot of such material, although I don't think there are any unreleased Jem songs or episodes. That makes me think MLP probably doesn't have much either. ANimation was much harder work in the 1980s, you'd think they'd use or reuse as much as possible to save costs.