Question

Exactly define the number of hydrogen and hydroxide ions at pH 6. What about pH 8?

Answer 1

In what volume?

pH = 6 means H^+ = 1E-6M and OH^- = 1E-8M.
If we take 1 L, then we have 1E-6 moles H^+ and 1E-8 moles OH^-. There are 6.022E23 H^+/mole H^+ so 6.022E23*1E-6 = # H^+. OH^- is done the same way.