Well, depends on the size of the star and what else you have competing for food. I have two in my 75, an orange (I think) and a blue. They do just fine. You could probably do one or two more, but it depends on what else you have that lives off the rock and detritus.
IME, the smaller blues do better that the larger, older adults. Long acclimation and a well established tank are also a must. Of course, finding one alive in the store is the main challenge.
HTH,
Mike