If you are setting up a claims enterprise portal the user will be prompted which identity provider to choose if you have setup multiple providers. For example if you have LiveID, Google, Azure AD etc. This might be confusing for users. If you are only allowing one identity provider then you can set that on the relay party. This is a quick look how to do this.
You also have an other option which will create more configuration but you could create a site and a respective relay party, each relay party would have one identity provider and then you would provide the users a different URL for their respective identity provider that you are allowing them to login with. This is more setup but might provider a better use experience if you are allowing users a choice of identity providers.