Arcbound Tracker

• WOTC Official Reminder Textofficial rules
  Modular — This enters the battlefield with N +1/+1 counters on it. When it dies, you may put its +1/+1 counters on target artifact creature.
• WOTC Official Reminder Textofficial rules
  Menace — This creature can't be blocked except by two or more creatures.

• WWizards of the Coast2021-06-18
  Official Ruling
  Arcbound Tracker will consider spells you cast before it was on the battlefield. For example, if Arcbound Tracker is the first spell you cast in a turn, each subsequent spell you cast after it's on the battlefield will cause the last ability to trigger.
• WWizards of the Coast2021-06-18
  Official Ruling
  If this creature gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.

Rulings via Scryfall