If a Wizard would normally enter the battlefield with a certain number of +1/+1 counters on it, it enters the battlefield with that many +1/+1 counters plus one on it instead. If a Wizard would normally enter the battlefield with no +1/+1 counters on it, it enters the battlefield with one +1/+1 counter on it instead.

The creature gets the counter if it would enter the battlefield under your control. It doesn't matter who owns the creature or what zone it enters the battlefield from (such as your opponent's graveyard, for example).

The creature gets the counter if its copiable characteristics as it would exist on the battlefield include the specified creature type. For example, say you control Conspiracy, and the chosen creature type is Wizard. If you put a creature onto the battlefield that isn't normally a Wizard, it won't get a counter from Sage of Fables.

The effects from more than one Sage of Fables are cumulative. That is, if you have two Sage of Fables on the battlefield, Wizard creatures you control enter the battlefield with two additional +1/+1 counters on them.

If Sage of Fables enters the battlefield at the same time as another Wizard (due to Living End, for example), that creature doesn't get a +1/+1 counter.