In my experience, multiple troop modules increase the capacity by 5 each. 1 legion is required to build and gives capacity for 4 more legions, for a total of 5. If you have 2 modules, then 2 legions are required to build, and it gives room for 8 more, for a total of 10.

I do just fine with one module.  I use biological warefare.  The least harmful it don't wreck the planet like the others do.  I put mine on speed.  Two deep space support and some scanners.  Also I use adv trans.