Issue
You want to support HTTPS requests on your application that contains more than 100 custom domains.
Resolution
Most certificate issuers limit certificates to 100 domains due to historical browser limitations and performance reasons. But, the Automated Certificate Management (ACM) system supports SSL for more than 100 custom domains up to 1000. With this system, each domain will get it's own ACM certificate overcoming the limit of 100 domains per certificate.
If you do not wish to use ACM, you can manually acquire multiple certificates that support 100 domains each and then upload those manually acquired certificates.
Additional information is available in this changelog - https://devcenter.heroku.com/changelog-items/1872