License for Developers

License for Developers

Hi,
I am a freelance developer and new to Zoho Creator. License model is confusing. Could you please someone explain the license model for developers like me?

For Eg: 
I have 10 clients. They own the app. I work for them.
Which case will fit for my need?
1. I need to purchase one user Creator license.
2. Each of my clients need to purchase one license for me. So it will be 10 license.
3. I can use Developer Zone. So no license needed at all.
4. Is there any other option?