I know this may be silly, but I've noticed (playing version 0.19.0.x) that the fleet size seems to be 1 higher than what's shown. At first it's 30, yet I could have 30 bats and 1 construction bot; then when I researched +20, my fleet size became 50 and I could still have 51 various ships with me. I even individually counted them on screen, but the fleet UI does also show 51 ships total, even when the header toolbar says 50/50.

I cannot get 32 or 52 ships, that is when the pop-up comes up saying "you're trying to build more ships then you are allowed."