As alluded to in the comments, there is no generic "right" answer: the number will depend strongly on the nature of your thesis, the state of the literature related to it, and your personal scholastic style. Nevertheless, in the spirit of engineering approximations, I will supply you with some rough guidelines.
As a lower bound, I would be startled if it was possible to place most theses within the context of pre-existing work in less than about 20 references. In certain contexts such as an obscure corner of pure mathematics perhaps this might be possible, but in most cases work does not take place in a vacuum, and other people will have either used similar techniques or cared about similar problems in the past.
As an upper bound, I would be concerned with a thesis with on the order of 200 or more references that the student hasn't been doing enough work on their own original research and has been investing too much time and energy in building a literature review, rather than a thesis. Again, there will likely be cases where a thesis with an extremely high number of references would be reasonable, but these are likely to be unusual.
Notice the extremely wide range and suspiciously round numbers in my estimates: really there is no "right" answer for how many there should be in general, but perhaps these will help with some initial first impressions in one's thinking.