try to keep those on the motherboard to a minimum.  1 or 2 shouldnt be a problem.  but air flow and power draw are concerns.  Be sure to plug in any supplimental power.  Some boards have an 8 pin cpu + another 4 or 8pin cpu or 4 pin molex at either or both ends of the slots.  Some even have 6pin pcie plugs like gpus and risers take. 


But whatever you plug directly in the motherboard will be pulling power through the traces of the board.  Its why most rigs are built with risers...that and airflow airlow airlow.