Numa's triggered ability goes on the stack without a target. While that ability is resolving, you may pay {X}{X}. If you do, a second ability triggers. You choose the target Elves and announce how the +1/+1 counters will be distributed as you put that ability on the stack. Each target must receive at least one +1/+1 counter. This is different from abilities that say "If you do . . ." in that players may cast spells and activate abilities after mana is paid but before counters are placed.