As far as I know, there are about 50 references attached to the master's thesis in our school. After reading more master's theses, you will find that most of them are over 50, and of course more than 30 are written. In fact, this is also a matter of face. If you write less, the teacher will think that you read less literature and the depth of the paper is not enough. In short, I suggest you write at least 50 letters of recommendation.

Finally, good luck!