party size with another player

I can't test this in-game yet because I'm at work

How many henchmen can I have in my party if it's me and my friend.... 2?
I know when I'm alone I can only take 3 (for a total of 4 in our party..)

You can take as many henchmen as there are available. Depending on how far in the game you are, the maximum party size differs. Just know that you can only have 1 copy of a henchman in your party (you can't have two copies of Mhenlo in the same party, for example).

Now if you mean heroes, then your friend can take 3 more with him bringing your party size to 8.

It also depends on where you are. Certain locations have party size limits, it'll either be 4, 6 or 8.

The party size always has a maximum. In the starting areas that party size is 4(later increases to 6 and8). How you reach those 4 players doesn't matters. So that can be you + 3 hench, or you + 1 person +2 hench,...

2.

The max (you said) is 4. You make one, your friend 2 and that leaves two spots open for the hench.

It doesn't matter how many people you have in your party, the max size is the max size. It your choice whether to take people, hench or heroes.

Thanks for the answers all