Hey Ravish,
Assuming you are using the on prem portal, the answer depends on your portal version. If you are running the 3.5 (or older) version of the portal, also known as classic portal, it is limited to one cluster. In the new portal series (4.x) a lot of work is being done on support for multiple clusters. But again depending on the version the support varies. At CA World this week they were showing support for multiple clusters in one portal and future features where you could even select which API's would be published where. So be sure to keep track of all the developments on the newer versions of the portal if you are interested in multi cluster support. You can also vote and keep an eye on the related idea in the comunities: Connect portal to multiple gateways
Regards,
Michiel