The wiki of harem says:
"A picture wherein a character is surrounded by 4 or more other females who have visible affection to said character."
However, I think the threshold of 4 is an overkill.
Dozens of tagged images have a harem of 3 girls only: post #957236, post #895010, post #784645...
And, sometimes, 2 girls is enough to form the "harem" situation very clearly, like in these images: post #393792, post #261183, post #990749...
So, if no one objects, I'm going to lower that quantitative limit. In my mind, it's so simple: If the image got a harem, it should be tagged with harem.
Updated