You should put 2 fixtures on a 3/4 inches water line. If you put more than 2 fixtures on a single 3/4 inches water line, then you will face water pressure problems. We found out many people connected 19 plumbing fixtures to a 3/4 inches water line. These include 4 showers, 2 outside spigots, 6 sinks, a dishwasher, 3 toilets, a fridge, and more. They have faced a lot of water pressure problems after that.