That's correct. The Hobby plan allows up to 2 custom domains per service, as listed in our domains documentation.

Thats a weird limit

The cap is per service rather than per account, so each service in your projects can have its own 2 custom domains. On the Pro plan the default goes up to 20 custom domains per service, as listed in our domains documentation.

oh wait i thought it was for the entire account, cause i had to remove a custom domain from another project

The custom domain limit is counted per service in each environment, so a domain on a service in another project does not count toward the 2 allowed on the service you're adding to now. Removing the domain from your other project wasn't needed to free up space for this one.

If adding a domain still fails, the service you're adding it to probably already has 2 custom domains. You can see them in that service's settings, under Public Networking.