Its worth noting that by saying "20/20" that means you can have a maximum of 10 squads of space marines (each squad costs 2). Each squad can contain 9 space marines, which means you can be under control of 90 units, and thats just infantry. If you count vehicles (which also have a cap of 20) you can have 20 rhinos, or 10 dreadnoughts etc. All told, thats over 100 units under your control at any one time.
And they all have guns.