I had one of my largest number of head shot sessions last December: a whopping 42 people signed up in one very short but fast-paced afternoon!
I wont post all the photos here as they all pretty much looked like this lighting setup -- which was a strong mix of ambient light from a window behind me.
This was enhanced by a diffused Profoto D1 Air (roughly on camera axis) and another D1 in an umbrella on camera right. Very, very subtle. The aim was to get a consistent lighting across all shots as much as was possible due to the dark winter afternoon daylight in December.
And for some shots I stepped a little to the left and photographed the clients against the white wall directly behind them to get another look entirely. This photo is an an example of this set up. Simple!