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nbk has answered already, I have set up your tables in a SQL fiddle, this makes it easier to play around. Consider using this tool for any more questions, because that makes it easier to talk about table structure etc.
http://sqlfiddle.com/#!9/45220f/2
To elaborate more on the question/answer already given: You need to group by a column, so that all columns sharing the same value will be grouped to a single row in the result. Then you can use aggregate functions on this group, like count. There are other aggregate functions like sum (to add numbers in a group). For more information see the documentation at https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html .