This is another good topic for our Wiki thought as it gets asked often but the discussions are difficult to find. Housing and feeding vary but this is my current setup:
I keep my shore shrimp in a couple of bare bottom 10 gallon aquariums with large cascade filters, top off with fresh water as needed (not daily like my critter tanks) and completely change the water when I get a new batch (about once a month). I have found that keeping a small amount of plant matter and snails in the tank seems to help with longevity. I feed them marine fish food nightly.
I keep my fiddlers in a couple of 2 gallon tanks with nano filters. The tanks are only kept at 1/2 full and I keep lava or LR in the setup so they can climb out of the water. I fully change the water about once a week. These also get marine fish food but seem to do better if fed sparingly a couple of times a week.
You aren't going to find and ideal set up, so you have to decide what will work for you. As for how many you should keep, it depends on how many you feed to your animals, and how long you can keep the food alive.
Currently, I have a 7 gallon trash can plumbed into my ceph system in which I keep shore shrimp. Easy peasy, but that won't work for everyone.