You can’t sacrifice multiple creatures to put more +1/+1 counters on Undercity Scavenger or to scry more.

You choose whether to sacrifice a creature (and which one to sacrifice) while Undercity Scavenger’s ability is resolving. No player may take actions between the time you choose which creature to sacrifice, the time Undercity Scavenger has +1/+1 counters on it, and the time you scry 2.