When you're in your instance, the top of the page should show your login name with a gren arrow pointing down. Click on that and you will see 3 choices, My Account, Logout and 'About'. Select About, this is where you will see what subscription type you have.
Enterprise subscription allows multiple streams.
Regarding what can be added, Marketo uesed to have this on their website, looks like they have removed in favour of a sales call.