How do I work out the amount of H+ ions in a sodium hydroxide and water solution? I know the $pH$, pressure, temperature and concentration of the solution. Is there a method I can use to work out the amount of $\ce{H+}$ ions?
The pH can be directly related to the concentration of $\ce{H3O+}$:

$$\text{pH}=-\log{[\ce{H3O+}]}$$
$$\Downarrow$$
$$[\ce{H3O+}]=10^{-\text{pH}}$$