Welcome to Reef Sanctuary. This forum is mostly for SW reef systems, but some of us keep FW tank also. I'll try to answer your questions.
Typically in a FW tank you want about 2 inches of fish per gallon of water. So in a 33 gal tank this works out to 33 two inch fish or 22 three inch fish. This is a recommended maximum, and your usually far better off stocking the tank to about half of that. When you do the calculation, remember that fish grow and you need to allow for that. Many fish grow very quickly.
Ich in FW is a very easy disease to control. The easiest way is to raise the water temp slowly to 86 and keep it there for 4 days after you no longer see the disease. That's usually all you need to do, but if you have a really tough case, you can also use Nox-Ich or similar compound at the sane time. Nox-ich will stain the water, and lots of other stuff it comes in contact with, so you may want to use this in a quarantine tank. Nox-ich and similar compounds should not be used to treat scaleless fish such as loaches.
With diseases, prevention is a lot better that trying to cure them. Make sure your water quality is excellent, and the tank fully cycled. Make sure you fish get a good diet.