Bitcoin
lightning
plex
whatever else, how many can i run on a single rpi board?
You will not get a definite answer to your question, it all depends on the type of apps, how often and with how many users you use them, how big is their database and how much it loads the CPU.
Nextcloud with the basic apps and 1-2 users might be fine. Add many Nextcloud apps, image analysis and conversion, dozen of users syncing GB of files and you will surely slow down your system.
Same for Plex, 1-2 occasional users with a small library and no video conversion, fine, more intensive users, hubdreds of movies and transcoding and you will get freeze and unstability.
I had installed many apps in the first months and while it seams mostly OK with CPU not too loaded when I checked, I ended up with issues like Mempool and Invidious app frozen or other apps being very slow.
I think this app store thing is great but also missleading regarding the capacity of a RaspberryPi or even the new Umbrel hardware.
Now I stick with mostly apps directly related to Bitcoin and I run media apps on a dedicated machine (in the process of migrating to a new TrueNAS custom server).
I run on Umbrel: Bitcoin (core), Lightning LND, Electrs, Mempool, RTL, Thubderhub, LNbits, BTCPay Server, Nostr relay, Uptime Kuma and a couple other I fogot and it looks quite stable for months now but I’m the only user for all of this, not intensive and mostly not in parallel.
I assume you’re running a Pi 4, if so I run all of the apps you mentioned above! You may need to watch your CPU/RAM temp/usage in settings if you install a ton of apps, but it should handle these 3 fine.