There are two questions there. What is the difference between 'amount' and 'number'. That is a question about whether the thing you are asking about is 'countable' or not. Apples are countable, so you use 'number'. Sand is not countable, so you use 'amount'. The other question is about whether to use 'the' or 'an' - that is a question of whether what you are asking about is specific or general. That is a very difficult topic for learners of English. I suggest you look at some resources - such as http://www.englishpage.com/articles/a-an-vs-the.htm or https://owl.english.purdue.edu/owl/resource/540/01/ 'I need an amount of sand to fix this'. (any sand would do) 'The amount of rice you need for each person varies' (you are talking about a particular amount) 'The number of apples on the table is 6' 'a number of people have arrived at the station'.