How many elements of order $7$ are there in a group of order $28$ without Sylow's theorem?

By Cauchy's theorem the group, say $G$, has an element $g$ of order $7$. To the contrary, assume $G$ has an element $h$ of order $7$ not contained in $\langle g \rangle$. Then, $\langle g \rangle \langle h\rangle$ has size $49 > 28$ (why?), a contradiction.