I don't have enough experience with scalability to be able to speak about it.

Right now, we have close to 5,000 users on the solution with one of our clients.

Scalability shouldn't be a problem, at least for the size of network team that I work with. 

Right now we have between 400 and 500 users. This is deployed in a network that is actually about 500 users.

We don't plan to increase usage. We're already using it as fully as we can and we don't have any more room. We might look at all the Microsoft programs that relate to office 365, like the ATP, because there is a difference between the ATA and the ATP. We are going to look and see whether there's any gaps that we can close. I think if you compare ATA with the actual ATP it's basically the same functionality. One is more on-premises versus the cloud. Since we are progressively in moving to the cloud, we might look at the ATP, which integrates better with the Microsoft cloud.

It's very scalable.