Upwork accounts are not resident on "devices."
Upwork accounts are maintained on the Upwork.com site.
If you create an Upwork account, you can use that account on various mobile devices such as phones and tablets, as well as on desktop computers: The desktop computer in your home office, in your living room, at our cabin, and also at the office space you rent. And also on the desktop computer you use while visiting your parents.
There is no limit.
But you are always logging in using the same Upwork username and password.
If you go to an Internet cafe, you will use the same username.
If another Upwork freelancer, who has no connection to you, happens to use the same computer at the same Internet cafe, to log into their Upwork account, that is okay.
Upwork does not limit the number of users who connect from the same computer. But they DO monitor IP addresses and log-in activity, and they have a lot of things they do (that they don't tell you about) to detect fraudulent activity. They are interested in finding freelancers who try to create more than one account. When they find those people, they remove them from the platform. Having two different users log into Upwork from the same IP address or same MAC address (same device) is not a violation of any Upwork rule. But certain patterns of activity might attract the attention of Upwork's automated fraud detection algorithms.