Hi Matt,
There is no set amount of websites you should be using. As long as you have the main ones covered you'll do just fine. You are more than welcome to use as many as you want.
I only use the main ones, just because it's easier to manage. There just isn't enough hours in the day to be using them all and constantly updating them. (I just don't see the point).
However,
http://ping.fm/ does offer a free service, where you write a single message, then they automatically update over 40 social media sites at once, for you (or as many as you are using).
This service is worth looking into if you aren't already and definitely saves you a lot of time and effort.
Good luck!!!!