In the Research section (icon at the top of the screen in the middle), so long as you have one either Civil or Military Lab, you can go to the Logistics tab on the left (third one down) and research greater fleet capacity.

The top row is for Capital Ship crews, the bottom is for overall fleet capacity. Each level of the fleet capacity will significantly increase the number of ships you can build, but also begin to tax your credit, metal and crystal income. Capital Ship Crews allow for additional Capital Ships to be constructed if you also have sufficient fleet capacity (each ship uses 1 Capital Crew, and 50 fleet capacity, so they're pricey).

I think that's what you wanted to know...