I remember asking the CAA, many years ago, as to why I (as a student) need not carry the aircraft documents required under the relevant schedule of the Air Navigation Order.
The response was that they view a training flight that departs and returns to the same airfield - even if it lands away at one or more airfields during that day - to be a single flight and, as such, the aircraft documents were not an in-flight requirement.
That would suggest the answer to your question is - ONE.