TOPIC: Limit the number of fields to select a multiple option

Hello,

I understand that for a field could offer more options and for each limit the selectable number.
But I need a field that would be limited to the total number of selected options.

Ex: I propose a transport with two different starting points, my bus is 9 seats.
I want to suggest they select it depart departure or if they go to the next step. But I still have 9 places available so this is the addition of the two options that determines if there is still space in the bus.
Thank you to enlighten me.

Merci de m'éclairer

Hello. Please explain further... in English... as I don't totally understand what you are asking. Give a more detailed example, or better, give a link to where this can be seen. Thanks.