osu!mania is a game mode which can potentially allow for 8+ keys to be pressed at once (although this is very rarely required by the music), but most keyboards these days reach limits far before this. It can depend on your binding locations and more importantly your keyboard hardware. This poll is to find out how many keys people can press at once in average so we can get an idea of what should be expected in ranked maps.
Play > Special > osu!mania > choose an 8k map (or turn on the 8k mod)
Start playing and press all the keys at once when using your preferred bindings. See how many actually display on the screen as being pressed. Make sure to hold them briefly and confirm results before letting go. Depending on the order you press keys, you may be able to "roll" more than otherwise possible, so make sure to test more than once and take the minimum reading.
Play > Special > osu!mania > choose an 8k map (or turn on the 8k mod)
Start playing and press all the keys at once when using your preferred bindings. See how many actually display on the screen as being pressed. Make sure to hold them briefly and confirm results before letting go. Depending on the order you press keys, you may be able to "roll" more than otherwise possible, so make sure to test more than once and take the minimum reading.