there is no hard and fast rule to keeping the number of LS in ur tank. it depends on how well ur system is set up, how good ur filtration (biological, chemical & mechanical) is, swimming space for LS, size of LS, and many many more endless reasons.
most LS could well survive without a chiller if the avg temp of ur tank is not 2 high and dun fluactuate 2 much. there r exception though, deep water fishes, e.g. cathalina goby will need temp of abt 25 deg to thrive in. dun think u can acheive a temp of 25 deg without a chiller unless u switch on ur A/C full blast. if u dun wish to hv a chiller, a fan might be ur next best option.
2 cents.