>>374304669
You can play solo zombie maps without any other players, but you'll miss out on the majority of the gamemodes. If zombie maps are all you want then you'll never need to talk to another person.
I think a server is required to play custom maps (you can play the regular zombhies gamemode with no server), maybe pick it up on sale if it gets a huge discount in the event custom maps can be played without a server in a future update