Scenario A: Any hentai consumption whatsoever is too much. Maybe some open minded people would set the limit between 2 and 5.
Scenario B: Over 10 would be the LD 50 for excess at this stage.
Scenario C: Probably no limit. If the individual feels shame then breaking 1,000 may give them pause, otherwise they'll just ask to see your collection.
Scenario D: Depends on the scenario. If it's a problem for the individual to the point where it's getting in the way of living a healthy life then the "zero is easier than one" principle takes effect and any hentai consumption is too much. Otherwise see Scenario A, but they'll be less outwardly judgmental.
Extra Scenario E: See scenario D.