The greatest picks outside of lane to counter Амуму include Люциан, Афелий, and Кейтлин. These picks are all great at countering Амуму with high win rates over 57%. Even if these Амуму counter picks are not chosen in the same position as you, it's not a good idea to choose him into these matchups. They will likely win against Амуму.
Conversely, if the other team choses either Хеймердингер or Кассадин, Амуму mains should strongly consider taking him, as he easily beats these selections with win rates over 55% against both ofthem.