If you cast a spell with multiple {X} mana symbols in its cost, the first ability triggers only once.

The mana added by the last ability can be used only to pay costs with at least one {X}, but it can be used to pay for any part of that cost, not just the {X}. That {X} could be in a spell's total cost, an ability's activation cost, or a cost you're asked to pay as a spell or ability resolves.

The mana added by the last ability doesn't all have to spent on the same cost.