See http://opensignal.com/reports/fragmentation-2013/ for interesting data on just how many platforms might need to be testing for a typical mobile app.
Short answer: to many.
So what is a tester to do?
Well as my book talks about I am a big fan of combinatorial testing (you can google on that and look for the tool from NIST called ACT). It is possible to use math to test a large space, such as numerous platforms, in a very scientific approach.
If you are faced with having to test an app on a “good” set of combinations, see this link
http://opensignal.com/reports/fragmentation-2013/ and then my book of combinatorial test attack 32.