Options leqno, reqno for documentclass or exist another option?

Question

I consider these simple MWE

\documentclass[12pt,leqno]{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
     x^n + y^n = z^n
\end{equation}
\end{document}

With the option leqno we know that LaTeX puts the progressive number of the equation on the extreme left. With the option reqno at the extreme right of the page.

\documentclass[12pt,reqno]{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
     E=mc^2
\end{equation}
\end{document}

But when I have, for example a code with align option, knowing that the number can be either on the first or second equation, how come the leqno option is not active and the number is always put on the right?

\documentclass[12pt,leqno]{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
a &= b+c \nonumber\\
e &= f+g
\end{align}
\end{document}

1. How do I put the number of the equation between the two equations in the middle or at left or at right?
2. Are there only leqno and regno options with LaTeX? But is there, for example, the ceqno option, i.e. the possibility of placing the number next to the equation (at the centre to be clearer) or that it is always at the centre but in the middle of the two equations?

Answer 1

If you want a single number for the display, use equation not align

Note that there is no reqno option, equation numbers on the right is the default.

\documentclass[12pt,leqno]{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{split}
a &= b+c \\
e &= f+g
\end{split}
\end{equation}
\end{document}