Cisco Help with a networking project?

I'm currently a student taking Cisco 3 connecting networks and scaling networks. I need to design a network that's compatible with 4 locations and a headquarters.

This design is also for a hospital so it needs to be secure, and needs to be able to hold 500 users with an increase of 10% in 10 years. I've already got the design and schematics of everything using packet tracer.

My question is how many users can be placed on one Virtual Private Network?

I have at least 8 VPNs and I've done my subnets using 10.10.0.0 network.