what version of itunes would be suitable for my Mac version 12.6.3?

None. The iTunes app for Mac computers was discontinued and replaced when macOS Catalina was released. It cannot be installed on Macs that are running Catalina or later. See the following Apple article for information about the replacement apps: What happened to iTunes? - Apple Support

Whichever one is available, but it likely won't be called "iTunes" but "Music".