The question really holds two, and they have opposite answers. How many devices may you use? As many as you like — DBExam is not tied to one registered machine, and any browser can open your Oracle certification practice. How many may be signed in at the same moment? Just one: only a single session is ever live on your account, so signing in somewhere new closes whichever one was already open. Free to move between devices, then, but one active login at a time.
Those two halves are easily blurred, so take each on its own — first the device side, which is open, then the session side, which is the one real limit.
Any Device with a Browser, No App to Install
You reach DBExam the same way everywhere: through an ordinary web browser, with nothing to download and no native app on any platform. There is no list of approved devices to register and no per-machine limit, so a desktop, a laptop or a tablet all reach the same account, and you move between them as freely as your day requires. Whether a small screen is a comfortable place to sit a full Oracle exam is its own question, and the answer sits on the Can I Use DBExam on Mobile or Tablet? page; here the point is only that the browser, not an installed app, is the door on every device.