How do I setup my iPhone 15 Pro to take pics at 48 mega pixels?

iPhone 15 Pro - Technical Specifications - Apple Support

9to5Mac – Here’s how to use the full 48MP iPhone 15 camera

In addition to what WheelieNick told you, note that you have to shoot photos using the rear (Main) camera, at a 1x zoom setting, to get 48 MP photos. Your Ultra-Wide and 3x Telephoto cameras have 12 MP resolution – and your 2x Telephoto camera is just your Main camera simulating a 2x Telephoto camera, at the cost of resolution.

Hi there

To capture resolution at 48 MP, go to Settings > Camera > Formats, then turn on Resolution Control or ProRAW & Resolution Control.