The official also said SpaceX might extend the limit on Falcon 9 booster flights beyond 20, the number at which Falcon 9s are currently certified for Starlink missions. The limits are lower for flights with customer payloads.

The current limit is 20 flights, but SpaceX might recertify for 25 or 30 flights depending on how boosters hold up after 20 flights. I believe the limit was previously 10 flights, which was then raised to 15, and then to 20.

I wonder where they will stop?

probably the ground