[Zeke]

You can have as many subs as you wish as long as you don't exceed the load capacity of the main.

In fact you may want to do that now that AFCI breakers are required for just about the whole house.

If you ran out or room with a 200 amp panel, either you have a huge house or are running a server farm. Or you have a huge shop. You may want to consolidate a few circuits. 4 bedroom houses don't need more than 4 light circuits (generally speaking, of course).