The maximum number of participants allowed in a boot camp before it is too big, is dependant on several factors. Safety, exercises and equipment used, size and area that the boot camp is being held and the comfort level of the instructor. All of these factors need to be taken into consideration to ensure a safe and successful boot camp. My current boot camp, an indoor/outdoor boot camp, is limited to 25 participants.
I think that as with any group training that “too big” is a product of what you’re doing (i.e. how complex the workout is), how experienced your group is (i.e. a group consisting of newer members who may not know the particular exercises or what to do, needs more careful monitoring), the age of the participants (i.e. of course kids need more monitoring than some adults), and the ever important trainer-to-client ratio that you’re comfortable maintaining without adversely effecting client safety, your ability to be effective etc.
A lot of factors to consider.
I hope that this helps.