at least 2, but if you get smaller ones, i would use 3, for better coverage, with less aggressive movement in certain areas(personally i'd go with 3), unless you have a sump, then probably 2 having added circ from you sump too,
the key is to have good circ(i like 10 times your gallonage per hour, ex. 550 gallons + per hour for all combined sources) with enough sources to not have any dead spots in your tank
HTH