Since pH is a logarithmic scale, a change in pH of 1 results in a ten-fold change in the concentration of H^+, which would be a ten-fold change in acidity/basicity . This is because how acidic/basic a substance is can be determined by the concentration of hydrogen ions. The more H^+ ions present, the more acidic the substance is, due to the fact that acids donate H^+ ions. On the other hand, bases accept H^+ ions, and thus the lower the concentration of H^+, the more basic the substance is.
You can calculate the concentration of H^+ from the pH and the equation pH=-log[H^+]. Rearranging, we get [H^+]=10^(-pH)
So for a pH of 8, we get [H^+]=10^-8
For a pH of 12, we get [H^+]=10^-12
10^-8/10^-12=10^4=10000 times less H^+ ions and thus 10000 times more basic
