Not sure if you've gotten a game yet in or not... but if not I'd be happy to answer that question for you.
There is a set number of ships for the game, but the amount each player has each round depends on how those set numbers of ships are played and dealt out each round.
An example is, with 3 players you have 9 ships in play. If two players both play 3 cards but 1 player plays only 2, then on round 2 when the 8 cards that were played are dealt out, the two players who played 3 cards have 2 or 3 cards that round and the player who played 1 has now has 4 cards to start the round.
Hope that helps!
Still haven't manage to get a game, so this might be something that's obvious when you play.
Why wouldn't I want to use 3 ships in my fleet everytime? Once the planet/moon have been mined, the used fleets to back into the draw deck don't they, so everyone's always going to have enough cards available to draw back to 3. So why would you ever use less than ships and be missing stats on a particular planet?