How many collective nouns this sentence contains The study group was interested in the behavior of the audience during a boring speech?

There are no collective nouns in the sentence. A collective noun is determined by its use. A collective noun is a word used to group other nouns in a descriptive way.

The nouns 'group' and 'audience' are often used as collective nouns, but in this sentence they are not. The nouns 'group' and 'audience' are not describing anyone.