You only need I layer box per every 3 hens. Usually hens start laying around 23ish weeks old but some breeds mature slower, like silkies & Cochins. My Cochin didnt lay until she was about a year old. You may want to look up laying ages for your different breeds to see what the average is.
I had this same dilemma,the only hen that was laying was my 6 yr old silkie then bang! I've got eggs coming out of my ears,39 in one week was my record,93 in a month. I try to let mine out as early as possible,14 hours of daylight helps to bring them into lay.
I have buff brahmas and was told they don't start laying till at least 8 months and some people have said there's didn't start till 10 to 11 months so I'm patiently waiting make sure to get some nest boxes set up that's really important to.