A good swoopy car, but take off the body and the complete lack of driver safety provisions becomes apparent. No structure at all in that area, but I guess as there were no seat belts, the driver wasn't expected to be hanging around in the event of an accident.
These Tipo 412 sports cars were built in 1938, after the rules for GP reduced engine size to 3 litres. Rather than discarding the 4.1 and 4.5 liter V-12 engines, the company built sports cars around them. They were raced a few times but development stopped with the onset of the war. The cars survived and continued to be raced successfully into the early 1950s. Lots more discussion here.
It's not certain how many were built. It's rumoured there could have been as many as 4, but most Alfa historians agree that there were just 2.