Connie,

I don’t think there are any technical limits on how high it can fly. FAA rules in the US limits legal flying height to 400ft, and I’m sure that altitude could be achieved by the Crazyflie – although you probably wouldn’t be able to see it at that height anyways. I doubt I ever flew it past 100ft before I started to lose track and brought it back closer to earth.
As for self-stabilizing flight, the built-in algorithm has stabilization to keep it upright while flying, but not to hover in a set position if that’s what you’re looking for. There is an altitude hold mode that will keep it at a set height, but with no GPS it will drift around a bit.
I hope that helps!

Thanks and have a great day!