I have looked over the master and PhD thesis of several past colleagues, and it seems that their thesis section are all very organized and does not deviate too much from the standard literature. In fact, I was not at all surprised by their selection and could recognize most of them (if their work was similar to mine). One of my colleague's citation mostly contained widely cited papers from well known conferences.
However, I work in the very multidiscipinary field of "theoretical mathematical biology for engineering applications in computer science related fields". So I need results from a wide range of topics.
I have found that I need to cite obscure math references in order to prove my result. On Google, some of these papers have no or 1 citation. This sharply contrasts with the selection of citation from my previous colleague, which makes me a little nervous. I'm sure these mathematicians are famous in their field, but should I keep on digging until I find the (similar) result in a well known textbook?
Further, the text in this area is extremely abundant but no single source contain all the results. This means I have to continuously cite textbooks despite already citing prominent ones such as Winfree's "Geometry of Biological time".
All in all I will probably have over 80 citations. Is this too much?
There seems to be no guideline or rule to selecting citations. Can people working in engineering, physics, and math provide some insights on how to select/filter the list of citations for a master or PhD thesis?