How do I know which version to specify?
You don’t unless you have a market analysis. Pick a relatively modern one (like OpenGL 3) and go from there is the best suggestion I can give.
Does it depend on the depth of my OpenGL code (what shaders/functions used, etc…)
And is this necessary for macOS?
It’s not necessary, but it does help with rendering performance all around!
(all Apple machines support JUCE’s OpenGL, correct?)
Modern ones support 3 (at least), older ones may be limited to ES 2.0.
Have a look at the official OS X OpenGL support table for more information: https://developer.apple.com/opengl/capabilities/GLInfo_1075_Core.html