Am I correct that shifting out has to be done twice because
there are two shift registers?On another note, I'll be using 48 leds with 6 shift registers.
can the arduino power that much leds? If not, would it be possible
to light one at a time in such a high frequency that it looks like
they are all on?
Yes, this is shifting out 2 bytes each time to fill all 16 LEDs. I think if it were only one shiftout in each place, you'd have the first 8 turn on, then the second 8 turn on while the first 8 turn off, going back and forth.
The Arduino off USB can provide 0.5 amps, so at 10mA each, it should be able to hand the 48 LEDs.
Normally it's possible to light one (or more likely several) at a time at a high frequency so it looks like they're all on. But with shift registers, it gets a lot more complicated to do that.