12 to 15 undereach should be fine with a 4 week veg. The ones that are showing some yellowing may be getting over watered if they are behind and you are feeding them as often as the more healthyer girls. Give them a little more time to dry out between waterings to let them catch up and dry off their wet feet IMO. An extra week or two of veg may help with filling out the canapy to take advantage of all that light.
"If what shone afar so grand,
Turn to nothing in thy hand,
On again; the virtue lies
In struggle, not the prize."--R. M. Milnes.
Hugh Miller said the only school in which he was properly taught
was "that world-wide school in which toil and hardship are the
severe but noble teachers."