I don't have a nail salon by itself, but I have a spa with a salon area. I have five chairs set up and there are four employees that alternate shifts, so two at a time. I don't know if you'd get more with more chairs or if two at a time is the limit.
They will go to whichever chair Sims are sitting in who request their services. I've never seen all the chairs full, only a couple at a time, but if the limit is two employees that could be why.
I have a yoga area with two different employees who rotate teaching, and the student mats are often all taken during lessons. I also have 3 massage tables upstairs, which each have their own attendant. There's usually one or two being used.
There's also an area for meditation with lots of stools and poofs set up, but the only time Sims seem interested in those is when I have my Sim lead a meditation class. If they'd made a designated teacher stool or poof, then it probably would work like the yoga classes.
