If we're talking Mario, most usually new enemies are kept to a bare minimum per level (like 1-2 new enemies), then they are reintroduced in later levels to keep players on their toes.
Take Super Mario 3D World, for example. In that game, there was like one new enemy or obstacle for every new level, and sometimes the enemies or obstacles weren't seen ever again.
Take Super Mario 3D World, for example. In that game, there was like one new enemy or obstacle for every new level, and sometimes the enemies or obstacles weren't seen ever again.