Typically (not specific as it depends on your chassis and layout), when fans are at full speed, the sweet spot for the most number of fans recommended considering noise/temperature/price is:
FRONT: 2 fans (towards the case)
REAR: 1 fan (away from the case)
TOP: 1 fan (away from the case)
If you wish to fill up all 6 fan slots of your chassis, there would typically be no significant temp difference at full speed compared to just using the 4 fans as suggested above. But, if you can control (slow down) the fan speeds, using 6 total fans at slower speeds can produce the same amount of airflow as using just 4 at max. speeds, but at a much lower noise levels:
FRONT: 2 fans (towards the case)
REAR: 1 fan (away...