That depends. The more polygons you add, the less framerate you have left for other things. The more polygons, the slower the game will be.
Though, keep the number of meshes (geomnodes) as low as possible. You can check how much meshes are supported by your graphics card using this function:
I don’t know. Someone else might answer this question better.
Er, note that the value returned by base.win.getGsg().getSupportedGeomRendering() has nothing to do with the number of meshes you can render. This method returns a bitmask, whose various bits represent the various types of geom primitives that the GSG can render natively. This is a very low-level method, and is used internally by Panda to optimize the scene for a particular GSG; there is usually no reason to call it from Python; and for most purposes, the number it returns is meaningless to the application programmer.