It mostly depends on the amount of data your local network and ISP bandwidth can handle. You can own any number of Rokus you can afford to purchase. Once you have them, they each need to connect to the internet to retrieve their content. If you have several all trying to stream high definition content at the same time, it's quite possible to use up your available bandwidth.
Additionally, some providers, limit the number of devices you can register to the account holder and also limit how many devices can connect at a single time. Netflix indicates 6 is the max for my account, looking at the "Manage Netflix ready devices and computers" link from "My Account and Help" page. I think that might be the same for everyone, but I'm not 100% certain.
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