If you can locate a building code copy (try an architect or engineer's office), it should spell out minimum requirements for fire extinguishers. Mark's shop sounds like it exceeds the minimum requirement.

I have a 1A-10BC and a 10lb CO2 extinguisher right near the door in my garage, one under the kitchen sink, another next to the boiler, and another just inside the pantry door. Also carry extinguishers in both daily driver vehicles (I don't have one in my 1951 truck -YET - but it'll be there once it's back on the road). Another good practice on dry chemical extinguishers carried in vehicles is to take them out and turn them upside down and whack them with a rubber mallet occasionally to loosen up the powder inside, which tends to pack down with the vibration they see.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.