If multiple creatures enter the battlefield at the same time, Sharp-Eyed Rookie's triggered ability may trigger multiple times, although the stat comparison will take place each time one of those abilities tries to resolve. For example, if you control a 2/2 Sharp-Eyed Rookie and two 3/3 creatures enter the battlefield, Sharp-Eyed Rookie's triggered ability will trigger twice. The first ability will resolve, and you'll put a +1/+1 counter on Sharp-Eyed Rookie and investigate. When the second ability tries to resolve, neither the power nor the toughness of the new creature is greater than that of Sharp-Eyed Rookie, so that ability does nothing.