Do I need a VPN on a Mac mini

The The Built in Security  is all that is required.

I agree wholeheartedly with P. Phillips. Unless you're using a true VPN tunnel, such as between you and your employer, school or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites. 

why do you think you need one?

the main purpose of vpn is to have tunnel across the internet and appear on another network, I use it when I work from home to my job so I can access company shared drivers, database servers and printers and the likes

some people use it to appear to be in another region they really are to get access to media and services not present in their local region

some use it to try and trick sites from tracking them with cookies and the likes

you can read more here

Virtual private network - Wikipedia

i'm one of those that uses mine to access content not otherwise available in my region. (Canada) but there are a couple things to consider if you want to use a VPN for that purpose. first, you need to get one that you can disable when not in use because your internet speeds can take a significant negative hit when turned on. it can even negatively affect downloading macOS updates / upgrades. and secondly, if you are wanting to use it for security, don't bother. IMHO, it provides nothing in the way of security.

I was talking about windows which my work laptop runs

as in only use it when I work from home, don't use vpn when I'm at work

or when I use other windows machines not operating on my companys network

there are a lot of myths about the need for them and third party virus scanner and various clean up apps

both on windows and mac

Not sure if Macs need VPN as I do realize that Macs have stronger security than PCs

VPN is not security, for you anyway.

A VPN provides security to a network that you would need to connect from outside, like home to a company network.

A VPN does not provide any security to you. All it does is route all of your internet traffic through an anonymizing server. If you trust the company providing that server, then you would gain some privacy. However, there is absolutely no way to know if that company is just bundling all of your internet browsing habits into a nice neat bundle to sell to advertisers. You are required to believe them at their word without any proof.

macOS Monterey has a services which routes your traffic through a private relay service to provide some amount of anonymity. You can also use Hide My Email to sign up using a randomized email address that gets forwarded to your actual address. That way no company has your actual address. When you discover non-related messages coming in on that email account, you know that the company sold off or leaked your email address. You can then shut down that email address.

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Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.

I use a VPN on my personal Mac to access my work computer. It's a requirement of my university's IT department. But if your employer doesn't require one, no need.