How many extra participants should you allow for?

How many extra leaders should you anticipate for a training? I was expecting 11 so I did a couple of extra's and when I arrived I had 15 on the list, but then they just kept on coming and I ended up with 19 in this room:

Ok we coped but it was tight and apart from opening the windows I had to deviate from the plan further. I split the Promise fan activity and the marking up of the Whatever the Weather challenge of the 5 E's into 2 different sessions. Fortunately the Promise fan template had 2 fans printed on each card and I had a stack of risk assessment templates with me, so leaders could do a fan & immediately follow it with a risk assessment.
                                       

 In reality there were too many questions being asked to stick to the training plan and that was just fine because the training was meeting requirements by answering their questions. On reflection we did cover most of what was on the training plan anyway. I did a couple of activities from "Learning to Lead" which is now back on the website. Hence the sweets for "Walking the line", the post-it exercise asking what gets the girls coming back week after week - turned up some surprises -the most frequently occurring post-it suggested Friendship was the main reason to keep the girls coming back. At the end of the session, when we checked what we had covered and how we would implement, a number of the leaders commented that they need to give more responsibility to the girls to choose and lead. Music to my ears. I felt sorry for the 2 leaders in the group who were obviously more experienced than the rest but the group drove the discussion in the main. Although the Educational Framework wasn't known about at all. Lovely lunch with other trainers and that was definitely my last training of 2016. What better way to sign out other than with chocolate!