One. I still get confused when installers etc ask me whether to install for "just me" or "anyone using this computer"!

Things get a little fuzzier on Linux machines where there is both my user and a an admin "user". I treat the other as an abstraction, and type "sudo" if it won't let me do something without invoking that. Sometimes there are problems where I need to install something with sudo, then can't access it as my normal user and vice versa.

On my own computers I typically have the two logins you mentioned, the user and the root account, since there's only one of me and only I use the computers. On the computer I recently had for work there were many accounts, since the sysadmins sometimes needed to SSH in. I also selectively added accounts for some of my coworkers as needed.

If I lived with any family, making non-sudoer accounts for them on at least some of the machines would seem like a pretty normal thing.

I make use of multiple user accounts for myself. Every computer has at least 3: root, personal, and work. I don't really understand why it's not more common to maintain separate work and personal logins but I do recommend it. One of the computers also has a local login for a family member, for when they want to use it.

For your bonus question: I run GrapheneOS and as of now I have 8 user accounts. This might be a bit much for most people. The idea is that my main personal and work accounts use exclusively open source apps, and any context which demands proprietary apps gets siloed off somewhere. Two of the user accounts have google services framework installed, but neither is logged in to the play store.

The reason that the phone is more complex is that I am essentially never forced to use proprietary apps on my desktops, there has always been a way to work around it. But not so on the phone.

I use a locked down kiosk user account when I’m responsible for driving slideshows or videos for events. I restrict the permissions so that if someone were to access the laptop, they can’t make any meaningful changes or pull up something inappropriate. I also change the wallpaper and account icon to match the event.

I could use a cheap throwaway laptop, but actually using my M1 MacBook saved me. One time I was running a slideshow that lead into a new years countdown, somebody decided the laptop was in their way so they unplugged the power cable and moved it. It ran on battery for 6-7 hours before I noticed and nearly had a panic attack.

It depends on the computer. Some of them have multiple local accounts because they're used by multiple people. Most of them have an account for me and for root. In most of those, actually logging in as root is disabled, though.

Two on Macs and Linux boxes. Particularly on Mac, I’ve had it where my main account won’t fully log in enough to be able to fix things.

Sure, I could boot into single user mode (can you still even do that on a Mac?) but it’s a pain.

Two, user and root, but I live alone and don't have to share with anyone.

On a mac and I have 1 user account for myself only.

Only mine. root only via sudo

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