I would say three or four
minimum (you MIGHT be able to do it with 2, but you will have to run some matches on a less-than-full battery), those that are not in use should be kept on a charger, use automated chargers that switch to float or trickle charge automatically.
As far as time is concerned, I'm not sure, but it depends on the current used to charge the batteries. Higher current = faster charge
but fewer cycles
so it should only be used if you have to.