Enough. Sorry, that isn't helpful but there isn't a hard number. Each vertex is a point at which the mesh can bend, each edge a spring that is the basis for the simulation. The mesh, viewed as wireframe, should have enough lines along which it can flex to assume the shape you want. Erring one the slightly mean side will speed the simulation, and may loo better for stiff fabrics; erring on the generous side will take longer and will produce a very thin. soft looking fabric.

dividing an OBJ should be done in a modeller - you could convert to SubD, set a number of divisions, and export as OBJ then reimport the result.

What is the current frame? I can't recall if it is necessary but I always move the frame indicator back to the beginning before starting an animated simulation.