This article makes the case that you should have at least 4.
Meanwhile, an interesting idea from the comments section:
After a tragic loss of a gmail account from hacking, I have a somewhat circuitous email management method. I use my yahoo email primarily for communication. I read my email in Gmail (I got used to the interface). My yahoo email gets forwarded to gmail, but stays on the yahoo server as a backup. Thus, I have all my email in two places online, if I ever loose one. I really don’t give out my gmail address, so there’s very little chance of it getting hacked.