You may obtain an additional API key through our Pricing Page.  Additionally, you may always use our sandbox (free of charge) for development and staging.
Multiple subscriptions are commonly used when:
- The software is shipped to multiple third-party's by the customer (eg. self-hosted software)
- To separate environments inside the company (development, testing, staging, production)
- To separate multiple independent production sites (EUROPE, ASIA, etc)
- To separate multiple teams inside the company working on different features or products