Mass Effect has a dozen companions at most, and I find that too much to keep track. Half a dozen seem to be my limit. Doesn't mean I don't care about the rest, but they feel certainly more 'disconnected'.

based on my experience with table top rpg's, which have more depth and more spontaneous interactivity between party members than video games do at present, the optimal number is probably four. need at least four for good interactions and variety. more than four tends to dilute things. but video games are not at the point yet where character attachment approaches that of tabletop rpgs.