I apologize for a delay in getting you this answer, busy Monday
I’m missing some information about your garage;
Does it have a drywall finished ceiling and no insulation on top of the drywall / attic shared with the rest of your house’s attic?
Is the garage’s ceiling open, only framed with no surface finishing material, space open to the house’s attic?
Are you planning to condition the garage; install a heating and / or cooling unit? Un-conditioned garages rarely have insulated ceilings and exterior walls.
Insulating the garage’s ceiling without conditioning of this enclosed space may not give you that many benefits. It might keep it slightly cooler in the summer and maybe a few degrees warmer during the cold season but if the high or freezing temperatures persist for a few days the benefit of that temperature differential is mostly from the fact that the garage’s interior is shaded by the roof and protected with walls and doors from wind and other weather conditions.
So, if you already have drywall installed on the ceiling and you’re not planning to use any heaters / AC in the garage, adding insulation on the ceiling will probably not change much. Just make sure there’s a good air flow, sufficient amount of ventilation in your attic.
If you have no drywall on the garage’s ceiling and this space is open into the attic directly above the living area, than you should finish the ceiling with drywall to create required firewall separation. Another solution is to extend the common wall between the house and the garage all the way to the roof and maintain an open ceiling for the garage area. You can check this link - garage firewall
Using baffles in between the rafters isn’t necessary at all; it all depends on the type of insulation used and skills of the installer. Properly installed batt insulation where there’s sufficient space left for the air to travel from the soffit into the attic doesn’t require baffles.
Let me know if you have more questions, just click “comment” below.