A RunSignup User can be the Primary Owner of a single Partnership at a time, however they can be a Secondary Owner of any number of Partnerships.
You can learn more about Secondary Partner Owners here.
You can switch between what Partnership you are currently using (Races created by you will be placed beneath the Partnership that is currently being used) by navigating to the Partner Dashboard here: https://runsignup.com/Partner/Manage
You can also access the Partner Dashboard by clicking the Partner item in the User Account dropdown in the top right hand corner of most pages:
Switching Partner Accounts
You can switch between Partner Accounts that you are either a Primary or Secondary Owner of by using the Partner Dashboard Sidebar.
At the top of the Partner Dashboard you will see the Partnership that is currently loaded. At the bottom of the sidebar of this page you'll see all Partnerships that you are a Primary or Secondary Owner of. You can click the name to switch which Partnership is currently loaded.