I think Level 20 is the max because there is a currency item called Mote of Light. You obtain the item from leveling up after reaching Level 20. So I guess that instead of gaining a new Level, you stay at Level 20, but gain 1 or more Mote of Light. I think this is how Bungie is treating the Leveling system. Making it so you gain a currency for leveling up instead of reaching a level cap.