2 Pilots
I know someone mentioned this before, but I will delve into it a bit more.
Despite the CRM, one pilot checking another, two sets of eyes (which are all vital parts), its the pilot incapaciation which is an issue.
Whilst it hasnt happened to me (touch wood), pilot incapacitation is a very real thing. Its the reason why we cant eat the same meals.
For some hypothetical numbers....how many airliners declared a PAN due to flight crew incapacitation globally last year?
Assuming that these were majority short-medium haul 2 crew ops.....now imagine that they were single pilot ops......a whole lotta big jet crashes.