Number of exercises and the learning of some subject?
Learning is a very complex process, do not expect to find precise answers like “5 exercises are required to learn a new subject”. There are many factors that affect learning, just to name a few:


Your existing knowledge.
Your engagement with the subject.
How you learn. Deep Vs shallow processing.
The complexity of the subject.
Your personal characteristics e.g. IQ.
Your current state, how much tired you are etc.


With so many factors involved is impossible to give precise answers.