My best advice - Don't worry too much about pH. Trying to change it will do more harm then good. Many tropical fish can adjust to a higher pH as long as they are acclimated correctly. If you can, do a slow drip acclimation so that the pH is risen slowly, along with the temperature. Just floating the bag only regulates the temperature. In 10 gallon tank, definitely stick with smaller fish. Neon Tetras are not hardy cycling fish, and if you haven't cycled the tank yet, stay away from them. Try a hardy fish for cycling, but keep up with your water changes. Try maybe some danios, rasboras, or if your set on Tetras maybe, try a couple Serpae Tetras or two Columbian Tetras.
Back to the original question - a 10 gallon tank should only house 4-8 small fish, even less if you are less familiar with water chemistry. The more fish you have, the more attention you will need to pay to water quality.