In my experience, there have always been 3 staff. I don't know that one of them has a designated "chef" role, but there does seem to be a lead F attendant and 2 other attendants. According to this thread, the lead in F is usually not the chief cabin attendant.

It seems like they keep it to 3 staff whether the cabin is mostly empty or completely full, having had experiences ranging from a cabin of 2/8 to 8/8.