It really depends on what stage of the model I am and what I'm painting for.
Currently painting very basic tabletop ready to get ready for a GT soon, so painting whole squads at the same time. Normally after the basic layers I switch to 2-3 models at the same time for highlights and shading washes. Finishing with single models for the small details like lenses, marks etc.
On my Tzeentch Arcanites army I do the skin 10 at the same time, then finish them up one by one, because I paint way better quality when going one by one and that army is my painting pride