MacOs Mojave - what version of excel for MAC do I need

You don't "need" Excel at all. There are a number of free or nearly free alternatives including but not limited to NeoOffice, OpenOffice, and LibreOffice. That's in addition to Apple's Numbers, which is also free.

I have had no need for Excel in many years.

For details, please read https://support.office.com/en-us/article/microsoft-office-support-for-macos-10-14-mojave-3593253c-95dc-4522-a8f7-8fb4fd16aff3