My Company (PAN) has 15 GSTINs, and we want to do the direct interface with the e-invoicing system. What should we do? How many IPs have to be whitelisted and test reports to be submitted?

You have to create the one client credential for your Company (PAN) after following the steps mentioned for questions ‘How to get the API Credentials for the Company on production for direct access?’ above.
Then create the API usernames for all the 15 GSTINs by linking them with the client id and client secret, generated in the above step.
You can whitelist max 4 IPs for your Company, and all your requests from all GSTINs have to come through them to the e-invoicing system.
You have to submit one test report for one of the GTINs.

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GST e-Invoice