At a time? One.
Well, that's normal household use. If you were doing something like setting up a bar or some TV bank with multiple TVs carrying the same content, then the Roku will still plug into a single device or piece of equipment. But it's up to the rest of your setup to mirror the content to other TVs. Just like if you connected a DVD player and wanted it on multiple TVs.
You'd need to find an HDMI splitter. Amazon has 1x4 splitters. And, of course, there is an upper limit to how far you can run an HDMI cable, usually topping at 50'.
DBDukes
Roku Community Streaming Expert
Note: I am not a Roku employee.
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