In situations like this, I typically charge $30 per person, with a minimum of $75. That way, if I’m training 4 people that day, I make $120. If one person skips, I make $90. If two people skip, I still make $75.
Everyones’ experiences are different, but I’ve found this to be the best for me. Instead of cancelling class because one or two people couldn’t make it, the class could keep going, and I’d still make a minimum amount that was acceptable to me.