Are there any limitations on how many children can one adult be responsible for on a plane? This is a useful piece of information in general, but in particular I am thinking about Wizz Air at the moment.
On the page Booking Children WizzAir specifies that up to 10 children can be accompanied by one adult:
Children between the age of 2 years and 14 years old are booked as children but must occupy their own seat and pay the adult fare
Wizz Air accepts children travelling together on the condition that there is a minimum of one accompanying adult (aged 16 years or above) per 10 children.
Children under 14 can only fly if they are accompanied by an adult who is at least 16 years old who will take full responsibility for the child.
Passengers of 14 and 15 years of age may travel unaccompanied but cannot be responsible for any passenger under 14 years old.
The accompanying adult must be booked under the same reservation as the child. If the child's seat on the flight is reserved separately from the accompanying adult the two reservations must contain the same customer name and address.
Children older than 14 years can travel on Wizz Air flights on their own.
In my opinion, how many children can travel (either alone or accompanied by an adult) would be a per airline decision, so you'd need to look up each airline's policies in regards children flying.