The rim on your bike is an important selection criterion for how stable, safe and comfortable you roll on the road or off-road. The connection between tire and spokes hub is responsible for the driving characteristics of your bike.
Of course, road wheels are first and foremost lightweight so that the different applications can be covered, for the MTB, of course, the rim should have maximum stability and as soon as it is aerodynamic, a high rim edge should be present. The right size of rim, whether 26 ", 27.5", 28 "or 29" should of course fit the frame and the fork.
In recent times, the rim width often comes into play, especially here a lot has happened. Especially MTB's are more often equipped with wider rims, so that the wider tires have a better fit. In racing bike area here is currently retraced and the usual tire width is now 23-25 ??mm and no longer 19-20 mm.
The number of spoke holes is simple, the more spokes the more stable (but heavier) is the impeller.