Can I plug in 1 common wire for more than 6 zones?

All my valves are connected to a single common wire.

Yes.

You could connect as many valves to a single common wire as you see fit. The reason we have placed multiple common ports on the unit is because many times valves come bundled in groups of 6. Electrically, it does not matter how many valves are connected to a single common, and you do not need to split your wire to go to both terminals.