You can and should use the same account (email address).
Just keep using the same account (email address) for all the Rokus you may buy - apps, Roku Pay subscriptions, and (some) third party authentications will be copied to each Roku, simplifying device setup and management.
... or do I have to have a second account and another monthly fee?
There is no monthly fee for a Roku user account. You say another, but are you referring to the charge for something like Netflix? Different channels have different restrictions on the number of simultaneous streams by a user. Depending on your Netflix account, you can have one, two or four streams at the same time. If you've paid someone for a Roku "user" account, you've been scammed and you might need some account assistance.
But as mentioned you can have multiple Roku devices on the same account. Just be aware that they will all have the same channels installed, and removing/adding a channel on one does the same to all devices linked to that account.
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