Just put in a checkbox option in the game.
This is supposed to be an adaption to D&D any GM would be able to acommodate between those party sizes.
The code can allready handle it , i just made a playthough with 2 of my friends and kaezeland & Us joined the party as a 4th & 5th party members in the tuturial.

Checkbok for partysizes please (alongside alot of other checkboxes for us to customize the game)

1. Party size of 4
2. Party size of 5
3. Party size of 6