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75 to pass which is equivalent to about 50 questions correct out of 100 in each part.
 
You might want to check the score list on this page, they were valid for the 1998 exams, the last ones to be released,

http://dentistry.uic.edu/students/exams/nbp1_1998/nbexam.htm

click on Score Conversion and check it.

It basically says:

Anatomy: 47 correct answers to get a score of 75.
Biochem/Phisio: 42
Micro/Pato: 44
Dental anatomy/Oclusion: 62 !!!!!!