I've seen four + the base pass inspection without qualms.
For what it's worth,I'd rather 'waste' an extra extension ring than try and cram conductors down into less space.
But, that puts me in the minority. I picked up this bias after becoming the favorite fix-it-for the inspection e-man at every firm I worked for. I came to hate 'tight' j-boxes.
Being the 'last man through' became part of my job routine.
The other stinker was being forced to clean up any warranty calls that stumped the other foremen. (Usually the bosses expected a foreman to fix his own stuff. But that didn't always work out. Then it'd be tossed into my lap.)
I leave this as a warning to other j-men: don't get my kind of reputation. Aim for Homer Simpson: he got all of the best jobs.