Will Apple include Mac mini 2014 late in his next OS upgrade after Catalina? Catalina is supporting Mac mini late 2014 models. But I want to know about the next os upgrade. Will Apple include Mac mini late 2014 in his next upgrade?

And one more thing is: for how many year will I get an upgrade of the latest macOS for my Mac mini late 2014.

I know it's very hard to say about it but then also any guesses.
If I take the reference of the following screenshot for all old models of Apple, your macMini should last for a couple of years more. After 8 years also, late 2012 still supports Catalina. 



In nutshell, if there are no any major hardware changes in upcoming macOS, I guess your 2014 will definitely support the next version of macOS.