So, I feel like the current standard originated in a time when implicates weren't as inclusive. Some being able to add 2 or 3 more tags if not more, and though not many abuse 4 tags being the minimum I have seen it.
I Think a 10 tags minimum would be fair, with the few exceptions of those zero content title images to series most all images should be able to get at LEAST 10 tags attached. As with how I said earlier, many tags having implications would probably bump those 10 tags up to 18 without extra effort from the poster which would end in better results using the search engine.
Updated by SnowWolf