We went with a group of 5 people and it was in my opinion the best size for the group, since it would not be expensive and you can hear and see everything the guide is speaking and showing.
We had an excellent guide by the name of Igor. He was articulate and funny. His english was very good and fluent.
We had no idea what we were getting into and I think it is the best way to visit the attraction - so no spoilers from me.
Wear warm clothes since its about 15 degrees celsius and footwear that is comfortable and can get a little bit dirty (sneakers did the job for me)
Definatley worth the money and visit. Highly recommendMore