The LaTeX-package ‘mathtools’ has the useful setting ‘showonlyrefs’ to suppress equations numbers, which are not referenced in the text.
AUCTeX’s preview currently ignores this setting. Here is a minimal example:
\usepackage{mathtools}
\mathtoolsset{showonlyrefs}
\begin{equation}\label{eq:1}
x=3 % <-- preview will display “x=3”, not “x=3 (1)”, etc.
\end{equation}
In~\eqref{eq:1}, ... % equation will be numbered, because referenced here