Honestly, I’d say the amount that you should have is the amount you can keep up. Some might disagree with me, but the key to success these days is social networking, and the more vectors you have to infect your audience with your meme/brand, the better your odds of success.
However, you must take care that you are actively able to ENGAGE with your audience on all of these. It’s not enough just to fire-and-forget a bunch of self promotion, you have to interact with not only your audience but also the other artists on these sites if you are to succeed. With how much presence you are trying to have, are you truly able to engage with the communities there? If not, THEN you might have too much.
My rule of social media – Don’t promote to your audience, engage with your audience. Make them your friends.