How many mastery level 5 can you have?

I'm assuming as many as there are champions?

I would imagine all of them assuming you had the time to play each champion that much.

I do wonder how it would show the "In Progress" at that point though since currently it just shows the highest level champions who aren't level 5.

Assuming you've got the time and dedication 129 or so. Which if you average 30 matches per champion would be around 4 k matches played to get level 5 mastery on all champions. Though that number will go up if they release another champion before you finish. I'd estimate around 3/4 a year at 20 matches a day to get level 5 on all champions.