I heat my garage in MN (similar to you weather wise) successfully
I mine for profit and my heating has evolved form a bunch of miners in a partially insulated garage to now fully insulated and even duct runs to distribute heat in 1000 square foot garage. Long story.

Size of garage and insulated vs. non-insulated all need to be considered.
I would say a typical 24 x 24 to 26 reasonably insulated garage would need at least 2 S9's to keep above 50 deg F most of the time. That a guess from my experience.