I was just re-reading the AAM book (from 2005?). In it, Brendan recommends that 10 shares is enough achieve a good level of diversification. I wonder whether people would agree with that view in today's circumstances.
Definitely 10 is not enough, though diversification is as much about the breadth of sectoral and geographic exposure as the number of holdings. Even with 20 holdings which were 'diversified' the idiosyncratic(stock specific) risk is still probably too high for most people.