Meschill, I don't believe there is any hard and fast rule here. It should be whatever you are comfortable with.
Just a couple of pointers:
1. Don't get too involved too quickly in too many sites. You will find that you don't have enough time to develop the types of connections that will be beneficial to you or your writing if you spread yourself too thin.
2. Remember that HubPages is looking for unique articles so you should not reproduce them on other sites. You may find that detrimental to your stay here.
3. Having said that, you need to determine how much time you have to dedicate to both writing the articles and the social networking required to make these sites work well for you