There is no published data because it depends on what is being found and how much as a percentage of the engine, if that makes sense. For example; in your case does the engine have chromed cylinders? if so you might expect a high count compared to an engine with steel or nitrided cylinder bores. The key to metal analysis is common sense, you need to find out what the metal is, (as you have done), then find out where it could have possibly come from. Also, how the metal has been treated in manufacture, for example; if the metal is 'case hardened' steel you could have a serious problem with a cam shaft or follower, even though the metal fragments might be small in number.
Bob