cPanel doesn’t offer an explicit option to limit the bandwidth usage for a subdomain.
Anyway, you can use a workaround for this situation. You(or your host) can create a separate cPanel account for the subdomain. Having a cPanel account for the subdomain will allow you to set a bandwidth usage limit for it. Like 20000MB, 2000GB etc. (Yes, it’s possible to create cPanel accounts for subdomains.)
If you are processing all requests via a single script, you can limit the bandwidth usage based on on-site visits. This will not be very accurate.
Related KB article: Use different cPanel accounts for main domain and subdomains