HotU starts you at level 15 and you generally reach level 26-28 by the end of the game, I think. The original campaign and SoU start you at level 1, but in the original campaign you will have reached level 3 by the end of the prologue. However the game has the whole higher level means less experience system so it doesn't really matter that much how many party members you have.
The more party members you have, including summoned creatures, the less experience you get.