The LaTeX command for norm is \| x \| (double vertical bar / norm).
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Write \| x \| 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.
Write \| x \|; for delimiters that grow with their content wrap them in \left and \right, or pick a fixed size with \bigl and \bigr, often the better choice.
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\left( \right)\left[ \right]\left\{ \right\}\langle \rangle\lfloor \rfloor\lceil \rceil| x |\lvert x \rvert\lVert x \rVert\left| \right|Delimiters in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \| x \| in math mode; it needs no extra package. For example, $\| x \|$.
None: \| x \| is built into LaTeX, so no extra package is required.