Thought Sponge

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2019-08-23
  Official Ruling
  Thought Sponge gets its +1/+1 counters at the moment it enters the battlefield. There's not a time that it's on the battlefield before its replacement effect has applied.
• WWizards of the Coast2019-08-23
  Official Ruling
  Use Thought Sponge's power as it last existed on the battlefield to determine how many cards to draw for its last ability. If that number is negative, you neither draw nor discard cards.
• WWizards of the Coast2019-08-23
  Official Ruling
  Thought Sponge checks the total number of cards drawn by each opponent, not how many were drawn due to a single instruction.
• WWizards of the Coast2019-08-23
  Official Ruling
  Thought Sponge checks how many cards were drawn by opponents this turn, even opponents who have left the game. The greatest number drawn by a single opponent is how many +1/+1 counters it enters with.

Rulings via Scryfall