As you can see different people have different opinions on this topic, just like practically every topic on this forum. That’s often because we have different experiences. Chicken come in different sizes and each chicken has their own personality. Some go broody a lot, many never do. I’ve had a nest hog, a hen that stayed on the nest for three hours to lay her egg and would not allow another hen to share the nest. I once saw a hen grab a pullet by the head and pull her off the nest so the older hen could have it. It’s pretty normal for me to see three hens on one nest at the same time, even with other very similar nests open. With some flocks I see most of the eggs in one or two nests. With other flocks I see the eggs pretty well spread out across all the nests. Each flock has its own dynamics and changing just one chicken can change those flock dynamics.
I find the larger the nests are the more chickens they can accommodate. I think that 4 to 1 ratio we often hear is based on full sized fowl using 12” x 12” nests. Mine are 16” x 16” and I have no problems with a 5 to 1 ratio with full sized fowl. Joel Salatin used 24” x 48” community nest boxes and says each nest can accommodate 24 hens.
In practically everything to do with chickens I normally advocate for more room, just to give you flexibility in how you manage your chickens. I don’t know your sizes, chickens or nests, but 6 nests for 15 hens is more than plenty. I don’t see any disadvantage in your having so many now that it is done, but you could have saved yourself some time and expense building them.