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.

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.

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