how will that number look when you first autoconvert via 2to3?
I did two migrations of >500k loc projects in an afternoon each, and admittedly some days of testing to gain confidence since there where few unit test. But i found it to be very smooth sailing.
I was very familiar with both projects, so that helped a lot.
EDIT: I also want to add that i did this using python3.5, when to ecosystem seemed to be at a sweet spot of dependencies supporting both 2 and 3 mostly. I guess if one has been waiting until now, the divide between library versions will be a lot bigger.
As a big user of logging and little to do with character coding, all of my admin/daemon stuff moved over with almost no changes necessary for 3.0 (actually ~3.3).
For some projects I did bigger refactors for 2.6/7 (exceptions) and 3.6 (fstrings).
My guess is that if you sweep GitHub for Python2 code and push it into Python3, that proportion would be under one percent.