Somewhere on the order of 60k images.
"Upgrading" the library appears to be a multistep process. There is a step before the application itself ever launches, then the Aperture window is available, and there is a background task "processing" images. This is where I'm stuck. 64bit mode just goes nuts consuming memory and then swap space until it complains that it's out of hard drive space for swap. More than 1000 pageouts per second, continuously. Grinds the machine to a halt. 32bit mode just sits there doing essentially nothing forever. No progress after 24 hours. Repaired and rebuilt my library and it processed another 20 images before doing nothing again. Then it started crashing on launch.
I've been seeing references to these kinds of problems elsewhere, but no solutions. What I haven't seen yet is someone saying "I've upgraded and it works great!", so I'm just wondering if the happy people are just quiet.
I'm pulling a copy of my library from Time Machine, and I'm going to give it another go.