The study guide indicates that approximately 1% of the weighting for the Salesforce Admin 201 certification is given to the topic of the AppExchange.
Given that there are 60 questions, you could expect around only 1 or 2 questions. Even though there is only a 1% weighting on the AppExchange, there are many powerful apps that can extend or enhance the power of Salesforce, so you should know the topic well.