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
