Well, assuming you want to power all of the wheels, the forces generated by mecanum wheels has been discussed
extensively. Similar analysis can be done for any combination and configuration of wheels.
For example, if using vector analysis on a 4-wheel partial mecanum drive:
-Two of the wheels can be modeled as described in the second linked pdf
-The other two will provide propulsive force only in the front-to-back direction. The 'only' side-to-side force will be friction, which will be whatever is needed to negate any forces acting on the wheel in that direction, up to the point where static friction is overcome.*
(*This is, of course, an approximation, YMMV)