both 'plenty of' and 'many' accompany countable nouns. Knowledge is an uncountble noun. 'many' does not go well with knowledge. 'A good deal of' knowledge - yes.
However, I would re-structure the phrase as follows:
Despite her great knowledge of English, she is afraid of speaking English in public. I think she must start to dare to do so if she is to ever improve her English communication skills.
Student or Learner