Two. Delta the emulator, and a clipboard manager.

I assume there will be a few more soon, though Apple still pushes for notarization and such, making things difficult for the lone hacker developing for fun. So I don't expect as many apps as once cydia had.

It's pretty cool that I can play Chrono Trigger and Pokemon FireRed Rocket Edition on the go though :) I'd love to see Epic bringing Fortnite to AltStore out of spite. :D

Epic has already announced they’ll be launching the Epic Games Store on iOS later this year.

https://www.epicgames.com/site/en-US/epicgamessweden

https://twitter.com/epicgames/status/1770500825166545305

If an alternate store can have a game emulator, could it also have e.g. an Android app emulator, which in turn could run Android app "ROMs" that aren't individually notarized?

You can run Linux software on iOS via iSH. Efforts to get anything more powerful on iOS is limited because JIT is restricted to web browsers only.

Then how do game emulators work?

Simple game emulators like Delta don’t need JIT since a static interpreter works. Anything better needs JIT to run at a reasonable speed

Is it not possible to add sources to the PAL store?

It is possible, but if sources have any apps that are not notarized (none of them are right now) they cannot be added.