In a game of Magic: The Gathering, there is no limit to the number of creature tokens players can create on the battlefield, but MTG Arena, the digital version of the card game, having too many creature tokens on the battlefield at any time can be problematic due to server constraints, and because of that, the game will be enforcing a limit to the number of creature tokens players can have on the battlefield at any given time.
In the December 2020 State of the Game, Wizards of the Coast has announced that each MTG Arena player will only be allowed a maximum of 250 tokens on the battlefield at the same time.
"If an effect would create a token past this limit, it instead does not; mana will be spent, cards will be tapped, permanents will remain sacrificed, etc., however, no additional tokens will be created if you are already at the maximum number allowed," Wizards explained.
Wizards used the card Scute Swarm as an example to explain how creating many tokens can stress the game's server. "While tabletop play can shortcut its way to a bug blowout, our game rules engine needs to resolve every single trigger every single time," WotC explained. "Creating that amount 1/1 green Insect creature tokens gets to be quite demanding and, at times, has caused enough strain on our servers that the scuttling of scutes was felt even in neighboring matches."
Wizards explained their decision to set the limit to 250 per player based on match date, "as most games will come to a normal and expected match complete before the maximum number is reached."
This is a welcome change to the game. At the point you have that many tokens, it probably won't matter much anyway. I've experienced games where the game slowed down because my opponent was creating too many Goblin tokens, so I'm glad they're enforcing a limit.
The max creature token limit will be enforced starting with the December 2020 update, scheduled this Thursday, December 10, 2020.