How is Ondu Cleric's Rally ability resolved when more than one ally enters the battlefield at once?
When Allied Reinforcements resolves, the two tokens enter the battlefield at the same time.  Ondu Cleric will see both ally tokens enter the battlefield, and will have its ability trigger twice.  The number of allies on the battlefield is checked when the trigger resolves, so barring any other allies on the board, you will gain a total of 6 life ( 3 for each trigger).

If you had a pair of clerics on the board, each cleric would see both tokens enter, for a total of 4 triggers at 4 life per trigger, giving you a total of 16 life.