Use \textup{\sout{<number>}}. But you should also use siunitx instead of making (wrong) symbols by hand: units should always be upright. Also “max” and “min” should be upright.
\documentclass[12pt,letterpaper]{article}
\usepackage{amsmath}
\usepackage{ulem} % strike through
\usepackage{siunitx}
\usepackage{textgreek} % to get a better “micro” symbol
\DeclareSIPrefix{\micro}{\text{\textmugreek}}{-6}
\begin{document}
[
\frac{\hat{N}} {O_{\mathrm{max}} - O_{\mathrm{min}}}
=
\frac{\qty{209.06}{\micro\volt}}
{\qty[parse-numbers=false]{(\textup{\sout{10777}} - \textup{\sout{5268}})}{\micro\volt}}
\approx 3.8%
]
\end{document}

Maybe you want to make your command for striked out numbers:
\newcommand{\sonum}[1]{\textup{\sout{#1}}}
ulemcommands are intended for use in formulae. Perhaps look at the cancel package. – Ian Thompson Jan 18 '23 at 22:12