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 with two additional +1/+1 counters on them.

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

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

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