A supernova is the explosion of a single star; so how is it that thousands of stars can "be born of" that one explosion (presumably only using the unspent fuel / lighter elements of the original)? Does that also mean the stars coming from a supernova cannot possibly become supernovae themselves?

For example, according to Wikipedia, the rosetta nebula has about 2500 including a couple O-type!

approximately 2500 young stars lie in this star-forming complex, including the massive O-type stars HD 46223 and HD 46150