>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?

Yes, but you can also add the passkey to your password manager so it's available on all your devices.

>Can I add another passkey on another device?

Yes.

>How many passkeys can I set up for a particular site/app?

I haven't really seen a specified limit on any sites, but also if you're using a password manager it's only 1 passkey for all your devices anyways.

> For now, I’m going to stick with LastPass and use Passwords; because no matter whether I lose my device or not or whether I’m on my own devices or not, I can be sure I’ll be able to get into a site/app.

Your passkeys would be in LastPass as well like your passwords, so arguably the same result regardless of which you use.

>Edit: One final consideration, my spouse and I share user/name passwords for some things (notably Pandora and our Amazon Prime account) since they don’t handle things like family logins well; how do both my wife and I use amazon or Pandora with passkeys? Do we each set up passkeys? How do I get her Pass if that’s not an option?

If it was me I'd add a second passkey to my password manager for your wife under a new entry, and share that entry to her lastpass account.

Or if she's not on lastpass, you could just copy the data from the passkey over to whatever she does use.