There is no specific limit on how many domains you can add to a project.
However, there are other limitations which could affect the number of domains you can add:
1) The routes.yaml file can only have a maximum size of 128 KB. This should allow for approximately 300 routes overall.
2) Automatically generated Let's Encrypt certificates have a maximum of 100 hostnames per certificate. If you add domains which are configured to use both the root and the www. subdomain, this limits you to 50 domains per project. This limitation can be avoided through the use of custom certificates.
You can read more about route and certificate limitations here.