Undercity Scavenger

• WOTC Official Reminder Textofficial rules
  Scry — Look at the top N cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.

• WWizards of the Coast2019-01-25
  Official Ruling
  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.
• WWizards of the Coast2019-01-25
  Official Ruling
  You can’t sacrifice multiple creatures to put more +1/+1 counters on Undercity Scavenger or to scry more.

Rulings via Scryfall