From a physical standpoint there's no limit on the number of environments you can have in Apigee. On-premises customers can add their own environments. Apigee Edge (enterprise) customers need to open a request with Apigee Support. The free Developer product only supports test and prod.
Side note: environments share hardware. So if you have a dev-test and prod environment in the same org and load test your dev-test environment you're on the same hardware (and therefore load testing) your prod environment.