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
10−810−12=104=10000 times less H+ ions and thus 10000 times more basic
