There are a couple of resources to help you prepare.
Check out Chapter 10 - Solution Sizing in "APM Best Practice - Realizing Application Performance Management". This overviews the whole process so you can answer the question accurately and confidently. Available on Amazon!
Below are links to a sizing worksheet. This is not updated for CA-APM 10.x - maybe someone has done that - but this will help you organize all of the data into a spreadsheet which will then give you the sizing - in terms of collectors, TIMs, etc.
The application survey is used to document complex applications or services. This can get you a very precise count of the expected metrics. The Sizing Spreadsheet uses a bit more of a SWAG for the number of metrics, based on the overall complexity of the application and number of instances.
For sure, the APM SWAT team can suggest some other documents, as appropriate.
Application_Survey.xls
EM - Application_Sizing_ALL.xls