I guess they work it out... take turns? They are kind of large boxes so sometimes I'll find two in a box together. I'm not saying 3 boxes for 16 hens is an ideal ratio, just perhaps the low end of the workable range. It isn't what I designed my coop to be, I began with 6 hens....
What got me thinking about this was housesitting for a friend, who has probably 14 boxes for about 20 hens. I've never seen such gross eggs as I take from that coop. Every single one is completely smeared with manure. Whereas my eggs come in the house so clean I rarely have to wash one. Of course there could be other reasons for this difference, but I did notice in that coop there are several low-ranked hens that are always in the nests though they aren't broody.
How would I know if the hens were really having trouble with too few boxes? I've been going with the assumption that I'd find eggs laid in corners, on the floor, wherever. But I don't.