Many small via vs few bigger via
This depends on what your end goal is but the answer is pretty much always use many smaller ones. 


If you are looking for better conductivity or better heat transfer, the amount of metal you get in a via is related to the number of vias and the diameter of the drill and how much it is plated. You will always get more effective metal volume (leading to lower thermal and electrical resistance) if you use more smaller vias that are inscribed in the larger via you would have used.
If you want to make your PCB cheaper to manufacture, you may want to minimize the number of drill sizes the fab will have to use. You should then use multiple of the size you used throughout your board to get you're desired conductance. 


The first point has diminishing returns however, and you shouldn't try to make vias as small as possible. Too small and they no longer use a drill, but a laser, and your PCB cost just quintupled.