Hi, let me try working on this from the bottom up approach (from demand).
The assumptions I'm going to make are:
1) Total US population: 320Mn
2) 50% split between Males and Females
3) 2 segments within Males and Females. Males - (The metrosexuals and the average Joes, whose demands are twice a month and once a month respectively). Females - (The fashionistas and the plain janes whose demands are once a month and once every 3 months respectively).
4) 10 seats in a hair salon, running at 50% capacity, taking 20 minutes per haircut and open 8 hours a day.
I'll start with Demand first.
Following the assumptions, we come out with total demand for Males to be 2880 (round to 3000 Mn for easy computation) and Females to be around 1280 (round to 1200 Mn for easy computation).
Total Demand in a year would be 4200 Mn.
On the supply side, we know each salon can serve 120 customers a day. (24 cuts a day per seat, working at 50% capacity, x 10 seats). Which translates to 840 customers a week, and (multiply by 50 weeks) 42000 customers a year.
Hence, the total number of salons in this case would be 100,000.
Doing a quick sanity check, that'll be about 3200 customers catered to, per salon in the USA, which i find to be okay.