The LeafCount command returns the total number of indivisible subexpressions in the specified expression.

$\mathrm{with}\left(\mathrm{MmaTranslator}\left[\mathrm{Mma}\right]\right)\:$

$\mathrm{LeafCount}\left(a\left(b+c\right)\right)$

$4$

$\mathrm{with}\left(\mathrm{MmaTranslator}\right)\:$

$\mathrm{FromMma}\left(\mathrm{`LeafCount[a[b\; +\; c]]`}\,\mathrm{evaluate}\right)$

$4$