The custom domain cap is 20 per service on Pro, and that limit isn't raised beyond 20 for setups that need many domains on one service. If your customers can use subdomains of a domain you own, a single wildcard domain (*.yourdomain.com) covers all of them with one certificate. Otherwise, the path for bringing many third-party domains is to manage them externally (for example through Cloudflare) and proxy that traffic to your Railway service.