I think three base could suffice. Building a fourth gets you more installation slots though.
I like to build my first base near Istanbul, the second one on the UA/RC border and the third one in Indonesia. Should you need a fourth base, Congo is a suitable location.
With three bases and radar towers, you should able to monitor most of the land on the planet, save for Antarctica.
With four bases and radar towers, you WILL be able to monitor EVERY single square inch of the planet - save for Antarctica, again. Who cares for those penguins anyway
First one is a mixed base, second or third one a research base.
Base defending is actually rather easy. Just make sure every single base you own has eight healthy soldier wearing combat armors.
You will need them, because - unlike in UFO:EU - aliens ARE going to find out where your bases are, even if you don't launch a single interception mission (that's what happens to me, at least).
(edited, typo)