I want to disable multitasking on my new iPad Pro. How?

Presumably your iPad is running iPadOS15.

Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new controls.

Key to accessing and controlling (or avoiding) multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.

More information about using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

Apple invite submission of comments and feature requests via its Product Feedback portal. If you have any suggestions as to how the multitasking functionality might be improved, this might be the most appropriate channel. For iPad and iPadOS:

Feedback - iPad - Apple

That is not a user option. The feature is a default part of iPadOS and cannot be disabled.