When the first ability resolves, you may put just one quest counter on Quest for the Gemblades. It doesn't matter how much damage the source dealt.

The first ability will trigger multiple times if a creature you control deals combat damage to multiple creatures, and/or if multiple creatures you control each deal combat damage to a single creature.

If there are no counters on Quest for the Gemblades yet, and simultaneously a creature you control deals combat damage to a creature and a creature you control is dealt lethal combat damage, the first ability will trigger too late for you to be able to activate the second ability and save your creature.