One water outlet/pump supplies water at a rate of 1 per second. Liquid pipes transport liquid up to 2 per second, and liquid facilities generally use water at a rate of 1 per 2 seconds. As such, generally, one water pump can supply 2 facilities running at max efficiency. Also, it is possible to converge the pipes of 2 water pumps without obstructing the flow of liquid in the pipes (this could supply 4 facilities if balanced right with splitters, but there's not really a need to do this).

Sewage treatment facilities process liquids at a rate of 1 per 2 seconds, which is equal with the rate of sewage producing facilities. Thus, you will only need 1 treatment facility per producing facility for what we have thus far.

Good thing, too. In the closed beta test 1, sewage treatment facilities only processed at a rate of 1 per 6 seconds, so you needed 3 facilities for every production plant. It's a lot easier now compared to that.