The LaTeX command for max is \max.
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Write \max 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.
In display math \max takes its full size; inline it shrinks to fit the line, and \displaystyle forces the large form.
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Use \max in math mode; it needs no extra package. For example, $\max f(x)$.
None: \max is built into LaTeX, so no extra package is required.