Latest OS for MacBook Pro (Retina, 15-inch, Mid 2015)

It can be upgraded to either Big Sur macOS 11 or Monterey macOS 12.

Personally, if it was mine, I would upgrade it to the current macOS 12.4 Monterey.

Also see: How to upgrade to macOS Monterey - Apple Support

or

How to get old versions of macOS - Apple Support

crm88 wrote:

Can I upgrade to OS 11 or should I stay at Catalina 10.15.7?

2015 is quite capable of running the current stable release of Monterey including bug fixes, is macOS 12.4

Keep your Mac up to date - Apple Support

Keep your Mac up to date - Apple Support