Hadfield: each capital ship requires 1 trained crew AND 50 supply.
Go to the third tab on the research window to see two rows of research. Research the top one to increase the number of crews that are trained to run your capital ships. Warning: it's expensive.
Then research your Fleet Supply in the lower half of the same window to bump up the maximum size of your fleet. Be aware this cuts into your production capability: the first level of research gives you 250 supply, but ALL of your resource production capability suffers a permanent 8% loss in efficiency - so if you made 100 crystal, 50 metal, and 1000 money every minute before upgrading, now you'll only make 92 crystal, 46 metal, and 920 credits a minute.
When the numbers at the top of the screen middle are, at minimum, 1 and 50, then you can build another capship. You can have up to 12.
-- Retro