What is difference between a pH of 8 and a pH of 12 in terms of H + concentration?

At a greater value of pH, [H⁺] is really low.

Solution with pH = 8 has more [H⁺] than solution at pH = 12.

Explanation:

pH = - log [H⁺]

10^-pH = [H⁺]

At pH = 8, [H⁺] = 10⁻⁸ → 1*10⁻⁸ M

At pH = 12, [H⁺] = 10⁻¹² → 1*10⁻¹² M

In conclussion, at a greater value of pH, [H⁺] will be really low.

Solution with pH = 8 has more [H⁺] than solution at pH = 12.