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.