In your case, for your described SaaS service you will need one extended license from Themeforest.
With the extended license, you can have unlimited paying users and each user can access the service within their own page, subdomain, or custom domains as well as long as all pages are generated from the central code that uses Metronic with an extended license. The extended license does expire so it's a one-time payment only.
We have a few thousand SaaS creators who used Metronic with an extended license and according to their feedback by spending 969$ for Metronic they saved up from ten to a hundred of thousand USD. Imagine how much a designer and frontend dev would charge for such massive work.
If you need any further clarifications please let us know.