Ships have levels like other units. They can reach at least level four.
There may be different types of ships, or the type may change with levels. One type is the Galleon.
Ships may level up via experience earned in combat like other units, or they may have to be created at a particular level, or increase in some other way.
A lvl 3 Haggar ship carried 24 units, while two level 2s and a level 4 carried a total of 75.
One possibility is limits of 10 units per level.