The number of spells you've cast is counted only as Aetherflux Reservoir's triggered ability resolves. For example, if you cast a spell, then respond to the triggered ability with a second spell, you'll gain 4 life total.
20-09-2016
wotc
The first ability counts the spell that caused it to trigger plus any other spells you cast earlier in the turn.
20-09-2016
wotc
Aetherflux Reservoir's first ability doesn't trigger when you cast Aetherflux Reservoir itself.
20-09-2016
wotc
The first ability counts spells you cast earlier in the turn even if you didn't control Aetherflux Reservoir as you cast them, and even if those spells were countered.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.