Ray Casting

I was looking up tutorials for learning how to do ray casting. This was the most frequently referenced site:
I read it and understand most of it. One quick question though. It has a section about doing walls of various heights, instead of having them all the same. There’s one part that says:

For a little more info, this tutorial uses a grid that’s “64x64x64 units” and each time a move is made forwards (or backwards, or left or right) you move 10 units.
So then, what’s a good limit for visibility? If you have a straight number that’s great, or, if you have a formula, that’s even better. Thank you very much for your help.