Restrict user to create only one db with a specific name

[Tiffany Thang]

Hi,
Is there a way in PostgreSQL 12 to restrict user to creating a database
with a specific database name?

For example, userA can only create a database with a name called mydb. Any
other names would generate an error.

If that is not possible, will it be possible then to limit userA to
creating only one database? Granting the createdb privilege would allow the
user to create any  number of databases which I want to avoid.

Thanks.

Tiff