Given $pH = 7.4$. Then $pOH = 14 - 7.4 = 6.6$. Concentration of $OH^-$: $$ \begin{aligned} [OH^-] &= 10^{-pOH} = 10^{-6.6} \approx 2.5119 \times 10^{-7} \text{ mol/L} \end{aligned} $$ In 1 mL ($10^{-3}$ L), the number of moles $n$ is: $$ \begin{aligned} n &= 2.5119 \times 10^{-7} \times 10^{-3} = 2.5119 \times 10^{-10} \text{ mol} \end{aligned} $$ Number of ions $N$: $$ \begin{aligned} N &= n \times N_A \\ &= 2.5119 \times 10^{-10} \times 6.022 \times 10^{23} \\ &\approx 1.51 \times 10^{14} \end{aligned} $$