Hmm. Leaving aside that an oratorio is not a stage work, wouldn’t it be better to use work type to say if a work was an opera etc.? “Number” is not an accepted term for musical parts of an opera or oratorio AFAIK. Typically such parts might be preludes, overtures, arias, recitatives, choruses, finales or just plain (instrumental) movements. For a “musical”, “number” might have a specific meaning (similar to aria) and my understanding is that it is this usage that has been borrowed for the new scheme for classical works. In other words, only arias etc. would be “numbers”, not recitatives or spoken sections. Given the potential for confusion, I think some style guidelines would be needed for this. Also, should the part types I have listed above be applied in some way - but as work type or relationship attribute?
Finally, oratorios tend to have Parts rather than Acts, so perhaps that should be a specific relationship attribute?