If Iridescent Hornbeetle leaves the battlefield before your end step, its ability won’t trigger at all. You won’t create any tokens.

If a creature enters the battlefield under your control with +1/+1 counters on it, Iridescent Hornbeetle’s ability counts those counters.

Iridescent Hornbeetle’s ability counts the number of +1/+1 counters you’ve put on your creatures, even if that happened before Iridescent Hornbeetle was on the battlefield, and even if those creatures are no longer on the battlefield or under your control (and, in some rare cases, even if those creatures aren’t creatures anymore).