Typically, it is one Garmin account per device. Although, many users, including myself, have multiple devices for various reasons. Having 20 watches associated with one central account would be challenging, especially considering the difficulty in analyzing a specific athlete's training metrics.
Is there a reason why you couldn't associate each watch with a specific athlete? Maybe some 3rd-party tool that will allow you to view all watches from a central device.
I've moved this thread to the Garmin Connect Web forum for better visibility and hopefully an answer to your question.
Not sure what exactly you are looking for, and what type of devices you own. But if you need to track the location of all the devices, you could perhaps try speaking to a Garmin representative - I see they offer Professional accounts for team tracking, though I am not sure whether it works for other devices than those with the inReach technology.
I agree with trux. 20 Garmin watches, even at a discount, would be a major investment. Assuming you purchased these through a marketing/sales rep, they should be motivated to find you a resource at Garmin to assist you with this.
