is there a limit of rokus I can have on my account? I would like another for my grandson, but I want to make sure it is ok .
Short answer. Yes, you can place multiple Roku devices on the same account (email address).
https://support.roku.com/article/360000321787
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By using the same account, each device is "linked" to that account. Channels/apps and subscriptions purchased through Roku are also linked to the account. Therefore, if you add or remove a channel/app from one device, it will be added/removed from all "linked" devices on that account.
Although I do not see an official maximum number of devices that can be linked to the same account, users have reported twenty (20) devices to be the limit.
Just use the same email address as you did with your other Roku device in order to link the Roku devices if you wish to share everything across all the linked devices.
Regarding subscriptions, this only affects subscriptions purchased "through" Roku. Many users prefer to subscribe directly with the provider (and pay the provider directly) which not only gives you the ability to also watch on non-Roku devices, but does not affect the Roku account as a whole. (you can remove from one Roku device and still have it accessible to another).
I believe you can have up to 25 Rokus linked to a single account. I've not been able to find this in writing anywhere though.
The closest I saw to anything official was a posting by @RokuJonathanD from 2021 (https://community.roku.com/t5/Roku-Developer-Program/How-many-roku-player-can-i-link-to-one-account/...) indicating 20 as a limit.
Certainly could have changed. Perhaps Roku should add the actual number to their support page regarding instructions how to link Roku devices to an account.
I likely just remembered wrong. 20 could very well be the number.
I only ever ran into this one time when trying to add a new Roku to my account. I received a bogus error message saying I was trying to exceed the (whatever number) limit of devices on my account. As this was only the 3rd device, and even if you counted older devices I had taken off the account the number would be far short of 20, so the message was clearly in error. Roku was able to reset something on my account and I've not seen the error message since.
I've always just thought they screwed something up when trying to get this $#&*!! Khoros forum software to correctly show my linked devices.