The number of ships you can put into a fleet is based on your logistics ability.
Different sized ships use up X amounts of logistics:
Tiny: 2
Small: 3
Medium: 4
Large: 5
Huge :6
Different civilizations start out with differing amounts of logistcs points (typically between 5 and 7). There are technologies (basic logistics, advanced, logistics, expert logistics, ultimate logistics) that add points to this. Eventually you could have up to around (if memory serves) around 50 logistical points.
So in theory, you could have 25 tiny ships in your fleet or 8 huge ships.
The game will be balanced (or will try to balance) such that you simply won't have that many ships floating around out there. I.e. if you have 200 ships you awnt to move together, we screwed up somewhere.