Must I purchase multiple channels ie. HBO Max, Netflix etc. for each ROKU device??
No. In general there are two ways to subscribe to different services - directly through the provider or through Roku. If you subscribe directly through the provider you can generally use the service on any streaming device, although there may be limits to how many devices can simultaneously use the service. If you subscribe through Roku you can use the service on any Roku device linked to the same Roku account (there may still be limits on simultaneous users). My opinion is that it's always better to subscribe directly with the provider, although some people prefer the convenience of having all their subscriptions in one place.
^^^ Me too!
Not only does subscribing directly with the channel give you the ability to see the channel on any platform supported by the channel, your account is directly with the channel and simplifies troubleshooting when there are problems. What you give up is consolidated billing from a single source.