The number of alpha hydrogens in butan-2-one, $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{COCH}{}_3$, is

Alpha hydrogens are on carbons directly attached to $\mathrm{C}=\mathrm{O}$. The $\mathrm{CH}{}_3$ group on one side has 3 and the $\mathrm{CH}{}_2$ on the other side has 2. Total $=3+2=5$.