.

From birth, I think two. Just another person, but obviously, for ethical reasons no-one's been able to test that.
#After a while though, one is sufficient because we can engage the social imagination and use the public concepts we imagine are available, even if they aren't. — Isaac

This is a good point to stress. Our Robinson Crusoe Cartesians like to take a result as if it were the given itself. I may end up a taciturn Heraclitus too wise for the company of others, but I started as a baby who couldn't lift my neck and (presumably) without even a concept that I was this self as locus of responsibility, tracked for what becomes 'my' promise-keeping and the reliability of 'my' wolf-reporting.