\pageref

Synopsis:

\pageref{key}

Produce the page number of the place in the text where the corresponding \label{key} command appears.

If there is no \label{key} then you get something like ‘LaTeX Warning: Reference `th:GrensThm' on page 1 undefined on input line 11.’

Below, the \label{eq:main} is used both for the formula number and for the page number. (Note that the two references are forward references so this document would need to be compiled twice to resolve those.)

The main result is formula~\ref{eq:main} on page~\pageref{eq:main}.
  ...
\begin{equation} \label{eq:main}
   \mathbf{P}=\mathbf{NP}
\end{equation}