The "Alpha Growth" Ancient Trait says "When you attach an Energy card from your hand to this Pokemon... you may attach 2 Energy cards." Does that mean I get to attach one additional Energy card or two additional Energy cards?
You only get to attach one additional Energy card, for a total of two cards attached at that time.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)