There's two ways of doing this, one in the table that's tricky and restricted to top 1 measures, so I'll describe the other which is hopefully what you'll need. If you work with the sample files that ship with Monarch, then open Classic.prn and Lesson10.xmod and you'll be good to go.
Create a summary with Composer as a Key field, and Sum(Amount) as the Measure. Double click on the Key field to get the properties, and then you'll need to deal with two tabs. Firstly, Sorting - sort it by Measure in Descending order. Then Matching - select the top n values you need.
There's a ton of options - how do you want to handle "All Others", for example - but that should get you started.