Hello!
I think that students should be consistent with their teachers, whether they have just one teacher or a few. I think it might be useful for them to have more than one teacher if each of their teachers is specialised in one of the skills they want to work on, for example, a student that wants to have informal conversation lessons AND an accent coach, but sees both teachers regularly because each teacher is best at that subject.
What I don't think is useful for students is to have a different teacher every time they book a lesson. Sticking to one teacher is best because that teacher knows you, your level, your needs and the things you like. The more lessons you book with a teacher, the more comfortable you can feel with each other and it will be much easier for the teacher to prepare the lessons with you. Sticking to teachers is the key to achieving a real improvement in the language.
I hope my thoughts make sense!