I use e10s for about a year and I have processCount set to 4. It worked almost flawlessly and content process crashed maybe 1-2 times in this time.
I enabled "Recommended performance settings" and Firefox changed processCount to 1. I have dual-core (4-thread) CPU so I think they don't use number of cores as base for this setting but make some A/B testing on users.
Check in about:config 'e10s.rollout.cohort' and 'extensions.e10s' and you will see that every user is put in some kind of testing 'bucket'.
I enabled "Recommended performance settings" and Firefox changed processCount to 1. I have dual-core (4-thread) CPU so I think they don't use number of cores as base for this setting but make some A/B testing on users. Check in about:config 'e10s.rollout.cohort' and 'extensions.e10s' and you will see that every user is put in some kind of testing 'bucket'.