But what would be the UI/UX of it ? It's quite visible in land fights when troops are trying to capture a building, how do you want to do it in naval warfare ? Where the garrison isn't visible on the map ?

Well, capturing a fortress was a major strategic objective in many ancient situations...

I am not sure is sdl, because shift+scroll works for Camera Rotation, that implies the command is being received by the Game. Even I updated libSDL2.dylib and nothing. I imagine is not mapped in the intel version, (but with trackpad it works =S) maybe if I map it to change batch size may fix it?. I was trying to add hotkey.gui.session.batchtrainingsize.increment = "Shift+WheelUp" hotkey.gui.session.batchtrainingsize.decrement = "Shift+WheelDown"  to the .cfg file but I don’t know very well the syntax. I made a second Partition with Windows in my MacBook and there it works (I have been not able to configure well, my monitor is 4K so when I use smaller resolution I get a giant cursor xD.)  

We need to debug why SDL is not picking it up. My guess is that it sends a different key code we're not handling on macOS EDIT: Seems to be working in the SVN version, maybe a recent SDL update.

I need to add something, because in macOS is not working for me the wheelUp/wheelDown for increment/decrement batch size (but is working for zoom and for camera rotation) How can I add it?