There can be an unlimited amount of difficult words per course but you will only be presented with a maximum of 20.
Once you have learned the word, you can add or remove difficult words by tapping the level and then de-selecting the golden yellow lightning bolt.
Once you have unselected a word, it will never reappear as a difficult word again. Alternatively, you can add or remove the word during your learning/review session.
What is Memrise Pro?
The Difficult Word Function explained in the website
What can you not do in the Android App?
How to sign up to Memrise Pro in the Android App?
Pro features in the Android App explained