You can embed complete graphs $K_1$, $K_2$, $K_3$, and $K_4$ on a genus $0$ torus (a sphere). The minimal genus of a torus on which you can embed $K_5$, $K_6$, and $K_7$ is a $1$. Then you need a torus of genus $2$ to embed ...
Is there a formula for any $K_n$, stating the minimum genus necessary to embed $K_n$ on a torus with that genus?