The LaTeX command for sharp (♯) is \sharp.
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Write \sharp inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
\sharp is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ♯ in a formula gives the same symbol.
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\mathscr{A}\mathfrak{g}\mathfrak{h}\mathfrak{n}\dagger\ddagger^\circ\flat\natural\checkmark\maltese\clubsuit\diamondsuit\heartsuitMore math symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \sharp in math mode; it needs no extra package. For example, $\sharp$.
None: \sharp is built into LaTeX, so no extra package is required.