Mortaccio Achievement in Vampire Survivors

This is cumulative across all runs. Skeletons show up in most levels of the game, and you'll see them as early as the first stage. Once you start being able to survive beyond 10-15 minutes, you'll see significantly more skeletons, and eventually the entire screen will be full of them at one point during the first stage. By the time you're surviving to 20 minutes in the first stage, you'll easily kill 3000 skeletons, possibly in a single run.