You can't easily chain Unicorn HAT unless you're bodging copper enamel wire from the end of one pixel chain to the next. They will happily run in parallel, but then that's just multiple sets of 64 pixels displaying the same thing.
I don't know what the hard limits are for the number of LEDs that can be driven, but it's far from unlimited. Users have suggested somewhere in the realm of 3000 LEDs, after which the library gives up the ghost. This is from way back in 2014 though.
One way to find out, I suppose, is to allocate 5k pixels and try to output the signal. Since WS2812s are write-only it's irrelevant if they're actually connected or not.
At those sort of quantities, though, power distribution is going to be a waaaay bigger problem.