You don’t choose a creature card when Savage Summoning resolves. Rather, it applies to the next creature card you cast that turn, even if you cast it at a time when you normally could cast a creature and even if you cast it from a zone other than your hand.

If you cast multiple Savage Summonings in succession, they’ll apply to the next creature spell you cast. That creature will enter the battlefield with that many additional +1/+1 counters on it.

Spells that would counter Savage Summoning or the relevant creature spell can still be cast. They won’t counter the spell, but any additional effects they have will happen.