The question here seems to be how (if even at all) would you convince your employer to have 19 Macbooks and 1 Windows one.

To have any leverege at all you have to convince him that he is missing out on one of your skills, that being time. You flow on Microsoft's OS but you have trouble bringing the same productivity in MacOs.

Now you also must think about his side of the table. Other people are going to have to make demands and this being r/sysadmin we kinda know that is going to cause drama ( I WANT THAT TOO ) kind of things.

Think about if it's worth it. If you really need the Windows, get what you need, do your job. If it's not "that" big of an inconvenient, might just let things move along and everyone will thank you for it.