A wave serpent is a unit upgrade, so it can only be taken in conjunction with a unit and it doesn't take up any slots. So you could have:
3 units of banshees each in a Wave Serpent
6 units of guardian defenders (max 10 in the unit) each in a Wave Serpent
and 3 units of Dark Reapers each in a Wave Serpent
For a total of
3 elite
6 troop
3 heavy support with
12 Wave serpents total
Note, I don't recommend the above list, but it would be legal.