You want to support HTTPS requests on your application that contains more than 100 custom domains.
Most certificate issuers limit certificates to 100 domains due to historical browser limitations and performance reasons. But, the new ACM system supports SSL for more than 100 custom domains up to 1000. With this new 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, we're also rolling out a new feature for SNI endpoint. This allows you to add more than one SSL certificate. You can acquire multiple SAN certificates that support 100 domains each and upload them through SNI.
To read more about this new update, please refer to our changelog: https://devcenter.heroku.com/changelog-items/1872