Multiple custom domains is possible, but are we talking about linking all those domains to the same Netlify website? For example, customdomain1.com and customdomain2.com should point to the same Netlify website? If yes, both those addresses will show the same website and would be redirected to the domain set as the primary one. Also, that’s limited to 99 domains per website, I suppose.
While creating subdomains is possible, you’ve not mentioned how you’re planning to create those subdomains.
If you can details about your use-case I’m sure the support team can work out a solution.
If you’re planning to create an individual website for each of the client, I think there’s a limit for 350 websites per account which exists to prevent spam and can later be increased by the support team.