Hi Brigitte,
I honestly don’t know, as I am not familiar with the other plugin. I could only check the configuration to see if those custom fields exist, but they don’t. There is probably a field that stores the capacity somewhere (maybe the “_hb_room_capacity” ?), but you might have to ask the other plugin author which one is it.
Once you have the name of that field, you could create a filter, something like this: https://i.imgur.com/VlZ0zbD.png
