Not really "a lot". When you think about providing absolutely best user experience, you gotta leave the engineering efficiencies out at the doors and live with the redundant API calls. The over optimization often comes with sacrificing the UX.
He could just request 100 posts first, halving the API requests, but the app would be less responsive.
Your comments seem to have a confidence level that their content doesn't justify.
That’s too late to be sending the request if you want the premium feeling of no loading times while scrolling, especially if you also want to do rich thumbnails of media.