I am a BSc Hons, and have just handed in my thesis (chemistry related), I did 11,000 words and got 35 references 50/50 books and journals. My supervisor said that it was a decent amount for a BSc level. Masters (or Mchem) it should be 50-60 with a minimum of 15,000 depending on the subject. As my subject was on a very new research subject, 35 was classed as a lot. So it varied depending on whos marking and the material available. However, with reading everybody else's responses try for 1 reference for every 100 words! you might be surprised at how much you can find!