The LaTeX command for ell (ℓ) is \ell (script l).
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Write \ell 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.
\ell is a math-mode symbol: in running text, wrap it in $...$. With unicode-math under XeLaTeX or LuaLaTeX, typing ℓ in a formula gives the same symbol.
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Use \ell in math mode; it needs no extra package. For example, $\ell$.
None: \ell is built into LaTeX, so no extra package is required.