So, so excited for this. Easily the most excited I've been for a game in quite a few years. Will be double dipping on PS4 and PC.
As far as monster counts go, I'm expecting it to be fairly high.
We have already seen 10 large monsters and they've only barely shown off a second area. Granted I'm sure there will be a good bit of overlap, but still, I believe we will see around 35 or so large monsters to begin with. I believe there will be 5 maps. Ancient Forest, Wildspire Wastes, an ice map, a volcano map, and a swamp or mountain map. All pretty safe bets.
In fact, there's a video floating around somewhere of someone looking at what appears to be a world map, and an ice area is definitely visible. I'll see if I can find that again.
With that said, 6 of the 10 monsters we have seen are all new. I'm sure they're eager to show off the new ones, but if we speculate there's even as few as 3 new large monsters for each biome, that's 15 large monsters right there. Then if we postulate that there are 4 returning monsters per biome, that's an additional 20 bringing our total to 35, assuming none of them are sub species. I think, given that logic, that is quite reasonable. Toss in sub species and elder dragons and I definitely think we could see 50 large monsters.
With all of the incredible quality of life changes and overall ... care they appear to be putting in to the game, I have no reason to suspect they would cheap out on the number or variety of monsters for us to fight.
Aaaaaaaaaaaaand since you asked, our new chameleon monster friend shares his skeleton and some animations with Khezu. Just sayin'.