I kind of use this general rule of thumb, take the number of people in your family and double it + 2. That number of hens...... kind of sorta, will keep you in enough eggs for your use, and some to give away.
The eggs are not constant, not a perfect egg a day deal. And some periods of the year, the production can really drop. The thing is, with 15 hens, depending on the breed of chickens you get, there will be times when you get lots of eggs, and then other times where not very many. In my humble opinion, 15 in not enough to set up a steady egg sales to very many people, and probably not enough to offset the extra feed that many hens are going to need.
I have 10 hens and a roo. Often I give a dozen of eggs away here and there as a small gift, or send them home with one of my kids. But in the late fall, early winter, there was a small spell where I bought eggs for just my home use. I am a baker, and my family eats a fair amount of eggs, and with the 10, I might keep another family in eggs, if I was darn careful in my own egg use, but not consistently.
With 15 hens, I think you are going to be in that expensive zone for feed, in which you generally get too many eggs for your own use, but not enough eggs to have enough sales to make the feed bill worth it.
That being said, I started off with 25, but some died. As the years have gone by, I have found a dozen suits me best........ so why do I have 25 ordered for the spring? Well that is chicken math.
MrsK