Roku likes their minimalist remotes. Some people want them to add numbers, but they haven’t even wanted to do that. A single remote for many Rokus seems unlikely to happen to me as that seems like a very obscure case, but anything could happen, and they could be working on some new remote right now, but won’t tell us until they’re ready.
Roku remotes use RF and/or IR. The RF does use the WiFi band, but is not normal WiFi. When using RF, they are paired to a particular Roku. I think you might be able to pair one remote for multiple Roku's, but then it will control all at the same time.
The only way to individually control Roku's is either with individually paired RF remotes, or through the app. There may be some 3rd party apps that offer some control (like Google Home), but again, that would be through a phone or tablet.
I don't know of any 3rd party hand-held remotes that support RF for the Roku. There is only one IR code, so it would control any non-stick Roku that is in sight.
The alternative suggested by @andyross of using the Roku remote app on a phone is probably your best bet. This works through your home network, not a remote-to-Roku wifi-direct connection, so it can access all Rokus on your home network and allow you to switch among them. The switch requires a couple of key presses -- you'd have to decide if you find this more or less cumbersome than juggling six dedicated remotes.
(And yes, Andy, you can control a Roku TV that way too.. 😊)
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The only universal remote I'm aware of that could control a stick was the now defunct Harmony Hub which operated much like the app in how it controls a Roku device. There are several Roku remote apps in the various app stores that could probably do it, but no handheld remotes.
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