How many pounds can a car have on it and still drive at least 25 mph?

All kinds of cars from Honda civic to a semi truck.
and also how much weight can it support and just move

Totally impossible to answer.

For how long?
What sort of damage?
Overloading a vehicle is illegal.

Two major problems involve (1) going uphill and (2) stopping when going downhill. Even then, there's the load capacity of the road and the bridges. When I was a wee lad there was a bridge that was (or at least seemed to be) a good shortcut for heavily laden gravel trucks. The weight limit was 8 tons. A truck with 56 tons sank right through it.