There is no single mathematical formula for polygon budget. It depends on the project. A robot in a Ratchet and Clank reboot will be different than a robot in Titanfall. You would have to look up their character art to determine how much you should be using. If you're just doing stuff for a portfolio, there are still some…
I would suggest look into Vehicle art for games (wireframes) the general guide for curved surfaces is that add enough tris for smooth curves and then terminate the loops somewhere they dont affect the shading (flat area mostly or even on curved area as long as you dont see artifacts in shading) Usually for vehicles from…