how many variables should be tested
The only way to test a particular variable for sure is to leave all other variables constant, and if a change in ONE variable creates an overall change, then you know that that variable caused it. But if you change 2 variables, and something changes, you don't know which one changed it for sure. So, one variable at a time. A full experiment can test many variables, though. better just to test one variable at a time that way one knows that any changes observed are accounted for by that variable.