I would think it's a judgment call following the question: do you have less bloom on a clump than has been typical in the past? I have been told that overcrowded clumps = reduced bloom, but I don't think there's a particular number of fans set in stone as the highest number you can have.

Another possible sign that a clump is too crowded - if you have dying fans on the outside of the clump after winter cold. Someone suggested to me that, when the clump is too crowded, the outside fans lose the protection from cold that the fans closer to the crown would receive. I don't know whether that's true or not.

I know someone who has an AHS display garden and who sells plants from the garden. For what it's worth, she doesn't list a daylily for sale unless there are at least 10 fans of it. That suggests that, at around 10 fans, you could replant some of the clump and still have a nice-looking clump.

It doesn't have to be 10 fans, of course. I once had a clump of BROWN WITCH that had 40 scapes one summer. Obviously, bloom wasn't affected by the huge number of fans. I didn't even have to divide the clump -- voles reduced the number in one winter to something like 7 fans. :(

Hope this helps!

I too go for the idea that they need thinned when they start blooming less or poorly. It seems like some virtually never need thinned and others need it regularly, Some daylilies I hate to thin because it takes them a couple of years to start blooming well again, so I put them off. Others don't seem to notice and bloom well right away so I am more willing to thin them. That is something you get to know over the years.
Brad.

Thank you I will leave them as they are for another year.
Arif