So let’s look at an example. Let’s say I have my Fathom Mage on the battlefield. And I cast Blessings of Nature, which puts four +1/+1 counters on my Mage. The Mage’s ability triggers for each +1/+1 counter placed on it. Since I just placed four +1/+1 counters on the Mage, its ability will trigger a total of four times, and I’ll be able to draw four cards. Seems like a good deal to me.
This also works if the Mage is entering the battlefield with counters. For instance, if I have a Master Biomancer on the battlefield and I cast the Mage, the Biomancer will cause the Mage to enter the battlefield with two +1/+1 counters on it. And since the definition of ‘placed’ includes a permanent entering the battlefield with a counter, the Mage’s ability would trigger and I would get to draw two cards (assuming the Biomancer’s power was still two).