At what altitude do you see the curvature of the Earth?

There are two possible answers to seeing the curvature. First you can see the curvature effects like this

Second you may want to know from what height you can see that the horizon line is not a straight line, but curved at the edges. Actually, if you put some efford in it, you can see this, just standing on the earth's surface. If you make a photograph of the horizon (of the ocean) together with a straight object, like a ruler close to it. Then zoom in and multiply the vertical distance by 10 (in photoshop) and you get something like this

See?

But if you want to see it with the naked eye you will have to climb up to 20 km for a start. (That is the mostly accepted rule of thumb).

The following video shows many aspects of showing the curvature, in both ways I mentioned above, and it clearly demonstrates how the appearance of the horizon changes with height. It ends with a double video showing a soyuz launch with a mounted camera. Earth Curvature Demonstration - By David Ridlen : David Ridlen : Free Download, Borrow, and Streaming : Internet Archive