There are limits in place on the random generation of maps, but AFAIK there is no (intentional) limitation in custom maps. So it'd probably just boil down to acceptable performance, out of memory crashes, or any bugs that may appear with large counts.

Asteroids probably do need to have a star associated with them.

Not sure what you mean about "Value".

The game only reads major race systems from customplanets.xml.

You should be able to start with mutiple planets owned in a custom map, though there's no way to do that for random maps that I'm aware of.