Let's ignore the fact that the concept of servers is largely a relic from past times on Retail and i'm going to take the wild guess based on Blizzards aggressive server mergers over the past years that you don't need a GM for every server, unless said GM should twiddle with their thumbs if they're not assigned to anything else.
Disregarding that i doubt you need to cover them 24/7 - i'm going to make another massive assumption and say that most players play around late afternoon - evening, not during the (late) night or usual working hours, hence that's where most spam is taking place and most people would care about its removal.