Hi bridgemans, 

You are able to have as many subscriptions with QuickBooks Online as you would like! When you first sign up you will create a User Profile which will be linked to your username, and once this has been done you can create as many files as you would like and link them to this same profile if you wish to do so. You can check this article for more information about the process to subscribe to an additional company after you have already signed up once, or if you have any additional questions regarding this process, feel free to reply in this thread and we'll be happy to assist you further. 

-Lucas