We had 18, yes 18 the 1st time we went to Pamps!
My daughter was the youngest at 7, the oldest was 60!
By the end of the week we were down to about 10-12, really learnt nothing after the 1st few days just followed the instructor down the green run over and over again.
Thank goodness we met Peter at Interski and used him for the next few years, came on in leaps and bounds.
If you can afford it have private lessons, it pays in the long run.
The Samakov ski school is one of the best in Borovets, the instructors are usually experinced and care about their customers, however the groups are often large especially at 1/2 term. I would expect 10-18 people in the group, any less and its a bonus. However the snow conditions are getting better, it should be great fun. If you get fed up have a few hours in the afternoon with a private instructor if the budget allows.