The Switch or Case action should work well.
Or you could also use the If Then Else action.
MACRO: Branch Based on macOS [Example]
**Requires: KM 8.2.4+ macOS 10.11 (El Capitan)+**
(Macro was written & tested using KM 9.0+ on macOS 10.14.5 (Mojave))
#### DOWNLOAD Macro File:
<a class="attachment" href="/uploads/default/original/3X/8/f/8f90857b75d097000fc1bf7d816b8af1c57ca788.kmmacros">Branch Based on macOS [Example].kmmacros</a>
**Note: This Macro was uploaded in a DISABLED state. You must enable before it can be triggered.**
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<img src="/uploads/default/original/3X/f/9/f941fbb53c0cafcdd9edbc62f761e21b71a41e42.png" width="596" height="1124">