A creature entering the battlefield with one or more +1/+1 counters on it will cause Enduring Scalelord’s ability to trigger.

If +1/+1 counters are put on multiple creatures you control (other than Enduring Scalelord) at the same time, Enduring Scalelord’s ability will trigger once for each of those creatures.

If you control two Enduring Scalelords, putting a +1/+1 counter on one of them will cause the ability of the other one to trigger. When this ability resolves, you’ll put a +1/+1 counter on the other Scalelord. This will cause the ability of the first one to trigger. This loop will repeat until you choose not to put a +1/+1 counter on one of the Enduring Scalelords.