What cable do I need to back up my iPhone in my new MacBook pro

hello Addy

Thanks for reaching out to the Apple Support Communities.

you have 2 solution

1. USB-C to Lightning Cable
2. USB-C to USB Adapter

Good luck & happy computing!

You can use the lightning to USB cable that's included in the box (the same that you use to charge your device), or you can buy a Ligthning to USB-C cable.

Read this article:

Using USB devices with your Mac - Apple Support

Regards

Giulio

https://www.apple.com/shop/product/MQGJ2AM/A/usb-c-to-lightning-cable-1-m

or

https://www.apple.com/shop/product/MKQ42AM/A/usb-c-to-lightning-cable-2-m