Sharepoint Integration with Multiple forests

Hi,

I have more than 10 entities and every entity is having its own forest and we have main forest hosting a sharepoint 2019 portal.

We have a requirement to let all users in all forests to be able to authenticate and access the sharepoint portal and we would like to do it without establishing any kind of trust between the main forest and the peers forests.

I am planning to do it by ADFS which requires to install an ADFS infra on every peer forest and on sharepoint to extend the web application and to create a new token issuer to every single forest...

I did the test for one of the forests and it is working fine how the limitation i am facing is that Sharepoint you can only do 5 extensions, is there any way to create more zones to be able to create an extension corresponding to every entity....

and Please share with me if there is a better approach to do it...

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Best Regards

Najwan