Cayth, Famed Mechanist

• WOTC Official Reminder Textofficial rules
  Fabricate — When this permanent enters the battlefield, put N +1/+1 counters on it or create N 1/1 colorless Servo artifact creature tokens.

• WWizards of the Coast2024-06-07
  Official Ruling
  If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
• WWizards of the Coast2024-06-07
  Official Ruling
  Fabricate doesn't cause the creature with the ability to enter the battlefield with +1/+1 counters already on it. For example, Cayth, Famed Machinist will enter the battlefield as a 3/3 creature, then its fabricate ability goes on the stack. Players may take actions (such as casting instants) while the ability is waiting to resolve.
• WWizards of the Coast2024-06-07
  Official Ruling
  You choose whether to put a +1/+1 counter on the creature or create a Servo token as the fabricate ability is resolving. No player may take actions between the time you choose and the time that the counter is added or the token is created.
• WWizards of the Coast2024-06-07
  Official Ruling
  Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the new token will work.
• WWizards of the Coast2024-06-07
  Official Ruling
  If you control no creature tokens when you populate, nothing will happen.
• WWizards of the Coast2024-06-07
  Official Ruling
  The new creature token copies the characteristics of the original token as stated by the effect that created the original token.
• WWizards of the Coast2024-06-07
  Official Ruling
  The new token doesn't copy whether the original token is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, color, and so on.
• WWizards of the Coast2024-06-07
  Official Ruling
  You don't have to choose every permanent or player that has a counter—only the ones you want to add counters to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
• WWizards of the Coast2024-06-07
  Official Ruling
  If you can't put a +1/+1 counter on the creature for any reason as fabricate resolves (for instance, if it's no longer on the battlefield), you just create a Servo token.
• WWizards of the Coast2024-06-07
  Official Ruling
  Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
• WWizards of the Coast2024-06-07
  Official Ruling
  When you proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.

Rulings via Scryfall