If I cast Savage Summoning, I can then obviously cast a card with bestow, which is uncounterable and gets a +1/+1 counter.

Does this use up the ‘next creature card cast’ of Savage Summoning, or do I still have permission to cast a creature card as though it had flash?

Savage Summoning's rules text:
Savage Summoning can't be countered.
The next creature card you cast this turn can be cast as though it had flash. That spell can't be countered. That creature enters the battlefield with an additional +1/+1 counter on it.

It's used up.

You cast the card using the normal casting process that's allowed by Savage Summoning, so you don't get another “shot.”

It's different if you had suspended a creature spell. Because Savage Summoning lets you cast a creature card as if it had flash, after you've cast it you can suspend a creature card with suspend. This doesn't use up the ability since you haven't actually cast the creature card; this also means you can suspend several cards during an opponent's turn (up until the point Savage Summoning's effect ends, that is, the turn is finished or you cast a creature card). And no, the suspended creatures won't get +1/+1 counters.

Daniel Kitachewsky
L3, Paris, France
Rules NetRep