How to put a symbol above another in LaTeX?

${a \atop \#}$

or

${a \above 0pt \#}$

Use \overset{above}{main} in math mode. In your case, \overset{a}{\#}.

If you're using # as an operator, consider defining a new operator for it:

\newcommand{\pound}{\operatornamewithlimits{\#}}

You can then write things like \pound_{n = 1}^N and get: