I was told 2-4 hens per nest.... but I don't think there is a hard and fast rule. The construction guy who helps us build our coop insisted on building 12 boxes - two rows of six. We have 11 pullets that just started laying... I think seven are laying so far. Six of the seven use the top row of nesting boxes... all six of them. My two Marans both use the top right nest, and only that one. 2 of my EEs use the top left three boxes, and my orpingtons use the top right three. No one uses any in the bottom row. One EE lays on the floor in the corner and I can't change her mind. Chickens get funny ideas, I guess.
So, in summary... seven laying pullets currently... they use 6 out of 12 boxes. One lays on the floor. Hopefully that helps you decide.
Edited to say: Congrats on your first eggs! 17.5 weeks is a nice early surprise for you!