Hello @kamlan19922 welcome to the community!
I agree the docs are a bit confusing when it comes to multi-tenant vs. multiple Auth0 tenants - Would you mind pointing me in the direction of where the paid tenant limit is mentioned? This might be something that depends on your subscription, but is generally only recommended if no other multi-tenant configuration will work. At this time at this time it’s only possible to configure 1 custom domain per tenant (Auth0 instance). This is a popular request so I definitely recommend upvoting this feedback request.
Hope this helps!