I have a Single Sign-On integration setup on my site. Can I use more then one SSO integration on the same site? Or am I limited to a single SSO integration?

You can have as many SSO setups as you like on a site, all to different identity provider servers. Pressero will create users on the site when a client logs in from one of the identity provider servers, and all users across all servers must have unique email addresses. If there is the same user on multiple servers, just make sure that the email address is exactly the same for that user on all the identity provider servers.
For more information about SSO please see the article Ch. 065b Single Sign-On (SSO).