Why does the Torah write "ha'Tzefarde'a" in the singular?
Rashi #1: In fact, only one frog emerged from the river, and, when the Egyptians began striking it, hordes of frogs began to swarm from it - the more they hit it, the more frogs emerged - until the whole of Egypt was full of frogs.
Rashi #2: According to the simple explanation, the Torah tends to refer to a swarm in the singular. 1
As we find later (in Pasuk 13), in connection with the lice (Rashi).