Clearly it's a recommendation from an instructor. More than two lessons a day early on in the course does tend to overload people, a lesson in the morning, break, rebrief and lesson in the afternoon is about as much as people can take to really learn and digest the information.
Obviously on nav flights later in the course you will do more than one flight in a day, but this is more consolidation of knowledge than actually learning something fresh.
No law at all on the matter though, nothing to stop you doing 10 flights in a day like most instructors would.