What exactly are you trying to do? You can simplify the gears by modeling the gears as cylinders, using the outside (or overall) diameter (OD). This will represent the amount of space required for the gear. Note that using this uption will show interferences between gears. I will typically place a sketch of the pitch diamter (D) to show the mesh location between gears.
The pitch diameter (D) can be found using: D=N/P where N is the number of teeth and P is diametrical pitch.
The outside diameter (OD) can be found using OD=(N+2)/P.
So for this
gear the pitch diameter would be 12/32=.375" and the outside diameter would be (12+2)/32=.438"
Hope this helps. In large assemblies, especially on lower end computers, it is better to make simplified representations like this and allow for the resources to be used where needed.