I recognize the point of your post is more about the lack of clarity and details around passkeys. That's real, and I don't really have an answer for that - other than, I think maybe the quest for making them simple and "just work" has maybe made them nebulous enough that we've wound up in the current situation where a lot of even technically savvy people don't really understand them. But I feel like answering your questions might sort of help explain why that's the case, so I'm going to take a stab at it:

> If I accidentally set up a passkey on my phone (let’s say I use Safari one day instead of my go-to, Brave), can I still log in without that passkey on other devices?

Assuming you have LastPass set up to be an iOS password manager, and it fully supports iOS' passkey implementation: when you create a passkey in Safari, it will ask you if you want to store it in LastPass or in the iOS Passwords app (previously known as iCloud Keychain). If you say LastPass, then it's up to them, but I assume it'll sync to all your devices - it's how 1Password works. If you were to accidentally say Apple Passwords, it'll sync to all your Apple devices automatically, and you can either use Apple's password browser extension on Windows, or you can use the "another device flow" I'm about to detail.

> Is there a way to ensure that passkey can be used on other devices?

As mentioned above, passkeys are intended to sync via your password manager of choice as the primary use case. If for any reason you don't have that passkey synced to that device, _and that passkey is on a mobile device with a camera_, most browsers will give you the option to scan a QR code with your phone. This kicks off a flow that will authenticate you via your phone's biometrics or passkey, then use Bluetooth to first ensure device proximity and then handle the authentication exchange. In the case of iOS, this includes any passkey-supporting password manager, so the passkey itself can be in 1Password; it doesn't have to be in the iOS password system for this to work.

When I first read the above, my hackles were raised given how well Bluetooth operates at times; but every time I've used it so far, it's been fast and flawless. Still, I can see a lot of scenarios where this might not work - e.g., the first one I thought of was a public computer at a library where Bluetooth might be locked down; corporate computers or remote servers could also be troublesome. As far as I know, passkeys don't yet have answers to those scenarios; other than to just use your password + 2FA as you would without a passkey.

As far as I know, both of the above apply to every passkey-consuming site.

> Can I add another passkey on another device? How many passkeys can I set up for a particular site/app?

This touches on your last paragraph, where it indeed could change based on the website. In my experience, every website where passkeys are fully supported - e.g., not ones that are using passkeys as a substitute for FIDO/U2F keys - has let me add multiple passkeys and have not _appeared_ to have a limit. I typically will create a passkey in both 1Password and Apple Passwords just to have a backup, and I can't recall any cases where that's been a problem. Still, I can't say for sure that isn't a problem on any website.

I went all in on trying passkeys when they started to be an option, and I don't have any notable regrets. For me, passkeys have generally worked well when the site is designed to use them well; and at no point have they been a _major_ hindrance. That isn't to say there are _no_ annoyances, though:

- Most websites that support passkeys tend to use them as a replacement for both the password _and_ 2FA, which makes them more convenient. However, a few - Amazon being the most notable I can recall - only use them as a second factor, which just makes them feel a little useless.

- A passkey can _also_ be used as the proof of identity, meaning you can log in in one fell swoop and don't need to enter a username or email address, which is IMO the best showcase for passkeys. Like above, this makes websites that ask you to enter an email address before letting you use a passkey also feel annoying.

- Most web browsers I've used support the QR + Bluetooth flow I mentioned above (otherwise known as Hybrid Transport or caBLE) without issue; Linux has been the odd duck out. Firefox doesn't seem to support it at all on Linux, and Chrome-based browsers do but sometimes are missing what they need and in that case don't show it as an option. Since I sync just about every passkey with 1Password this typically isn't a problem; the exception is the passkey for Apple Accounts, which Apple creates automatically, and (AFAIK) doesn't allow you to enroll your own. Apple Accounts are the only service I've found that does this, though.

- Some websites seem to only offer passkeys as an option if you're on a mobile device, or at least did so at the time of enrolling. eBay and PayPal I think are the two that jump out at me as having done this. Why they did it this way instead of simply detecting if the browser supported passkeys, I have no idea.

All of the above issues have gone down over time, so it's generally been a net decrease in friction over time. And, at least as far as I can recall, passwords themselves continue to be an option in every instance I've enrolled a passkey. So if you like your passwords, generally speaking, you can keep them :P