are you looking for more variety or more abundance?
if the latter, it is pretty easy. All you need to do is in the Main_EnemyAppearance.txt file crank up the 'Ship Repeats' for each entry and perhaps raise the chances a little bit.
... also adding more enemy Groups in the same file can give you good results, but needs a little more work. Depends really whether you wnat more single ships or battleGroups.
note that you'll have to do this for all 'Appearance Group Names'