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.

Aetherflux Reservoir's first ability doesn't trigger when you cast Aetherflux Reservoir itself.

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.

The first ability counts the spell that caused it to trigger plus any other spells you cast earlier in the turn.