How to impose a global limit on two webform number fields ?

Hello

I'm using a webform as an online reservation form.
Apart from the name, email phone number fields, there are two number fields to ask how many adults and how many children will be attending an event.

The thing is we want to restrict the online reservation to 8 participants (adults and children combined)
For example : It can be 8 adults or 4 adults and 4 children or 6 adults and 2 children etc..

Is there a way to use some sort of rule to impose this ?
thanks