How many is safe? I don't mean that facetiously - do you have a good understanding of a safe power limit on a source at hundreds of volts?

15mA is the limit in my brain. Im sure someone will chime in with a different number but its around here.

The thing to remember is the output impedance. You can have 1000's of volts at a node, but its sourced by a VERY high impedance output. so when you touch it with your hands (which is like 100kohm on a dry day), the voltage collapses because of the voltage divider your hand created.

But if the output impedance is low (transmission lines), well its game over.

There's a really good video on this:

https://www.youtube.com/watch?v=BGD-oSwJv3E

But even beyond that, the maximum power output will still be pretty limited. If you short the HV rails, it will almost certainly start to drop in voltage and raise in current quickly, but only to the limits of the resistance from other elements in series and the power source's output ability. I strongly doubt you could even make something dangerous from the Flipper, at least unless you attach a large cap, let it charge slowly, and attach a taser module.

Talking a bit out of my ass here as someone whose EE experience ended at graduation, but a good general rule is if you stay below 5 mA continuous, it is rather difficult to injure yourself. For extremely short shocks (like you get when you discharge a capacitor across yourself) you can get away with a lot more than that, though.

I would be rather surprised if a device that is powered off a lithium ion battery like the one in the Flipper Zero managed to seriously injure someone.

Yes. Someone else already did the hard work for us on that. GFCIs commonly trip at 5mA. Pretty hard to really hurt yourself below that.

Depends on the specific conditions but less than 2mA for AC should only be a tingle and closer to 5mA for DC will be a slight tingling.