It is totally your choice there is no restriction on the number of ad units you can use in a single app. The major differences will be in the approach
Single ad unit will have easier management but all stats will be counted under same ad unit and you'll not be able to make out which activity generated more revenue.
Multiple ad units will require more management in terms of creation and implementation. But you will be able to get more insight per activity in your app.
The approach you choose, again, totally depends on how you want to get it done.