The reality is it can't always be achieved. Example:
Student asks to fly every Sunday, so he/she is put with a Sunday only instructor. After 5 hours requests to fly on multiple days per week to progress faster, ends up with who is available on each day.
We aim for one instructor for the early exercises. By the time nav has started it can end up as much as three different. Using young restricted instructors, fresh out of the course, can work as one to one because often they will fly whenever asked. Unfortunately they not only need supervision but also lots of coaching on how to actually carry out the tasks!
We only work with very carefully chosen "mature" and extremely experienced folk who know how to work with each other both for the student's benefit and for many other reasons..