Song of Creation’s first ability is cumulative if you control more than one. It’s also cumulative with other effects that let you play additional lands, such as the one from Dryad of the Ilysian Grove.

Players can cast spells and activate abilities after the first triggered ability resolves but before the spell that caused it to trigger does.

Song of Creation’s second ability doesn’t trigger when you cast it because it’s not on the battlefield yet.

An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.