To win against the AI, you don't need any bases at all (see Zso's successful completion of the Nomad Challenge)...mostly though, it depends on your game. I expand exponentially on my starting continent until I either fill it up or run across some other faction.
If I'm alone, and fairly content with the number of bases I've got, I won't even bother with sea-colonies, but if I got stiffed in the land department, I'll expand right on into the sea until I've got "enough" bases (18-24 in this case, or however many it takes to fill up the continent using a "three-apart" spacing scheme). At that point, building additional bases is vastly less efficient than simply taking someone else's bases. Lousy as the AI is, at least they DO build some infrastructure, which is better than starting from scratch and having to ramp it all up yourself.
Of course, depending on the prevailing game conditions, you might find it in your best interest to fully colonize and develop a smallish island near you and give all the bases on it to the AI opponent who's in the worst poisition (assuming he hasn't been catty and declared a desparation vendetta on you). Not only will you gain an ally (and an easily controlled one at that), but you'll muck up the game dynamics for the rest of the AI, which is always amusing....
-=Vel=-