It depends on the wattage of the mat and the stat, for example if you have a stat with a max of 100watt then you could run 4 25watt mats. However I woudnt recomend it as its really hard to get the temps right for each, as my collection grows Ive just been buying more stats, the most I have on one is 3 but im not entirely happy with the temps.
As said u can run numourous mats up to the Max output of the state. But the mats do have to be identical brand an size, all need to be monitored an all need go be allowed to aclimatise before turned on.