How many Customers do each of the plans support

There is no limit to the number of customers (end users) supported on any plan.

How do we, as agent/s, segregate any of your products across multiple clients? In other words, do any of our customers have access to your platform to generate tickets?

Hi Evan! You can allow anybody to use your Zendesk, close it to all but the users you add, or restrict the use of your Zendesk to just users from specific email domains or within a range of IP addresses. You can find more information on working with these settings here.

While end-users may be able to use a "Submit a request" page in your Help Center, only agents will ever be able to access the agent interface to view tickets in your account. 

I hope this helps helps clarify how your customers can create tickets. If you have any questions on this, please let us know!