From FreeSpace Wiki
rand Rand (Arithmetic operator) Gets a random number. This number will not change on successive calls to this sexp. Returns a number. Takes 2 or 3 numeric arguments... 1: Low range of random number. 2: High range of random number. 3: (optional) A seed to use when generating numbers. (Setting this to 0 is the same as having no seed at all)
If you want to randomize a different number every time a given event is triggered, use rand-multiple.