From the 5 V pin, you can draw as much current as your power supply allows, as stated on eLinux:
Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board.
Note that for the Pi 3, the 1 A current input is inaccurate; the Pi Foundation recommend 2.5 A instead. pinout.xyz suggest that "with a decent power supply, such as the official Pi adaptor, you can expect to pull about 1.5A."
The MAX7219 datasheet states that:
Operating Supply Current: TYP 330 mA
So, with 8 LED matrices, you're looking at 330 mA * 8 = 2.64 A, which is probably beyond what your PSU can supply.
Also interesting: the documentation for the LedControl Arduino library, which goes into detail about the hardware decisions you will face. Note that the current draw will depend on the LEDs that you've chosen — as pointed out in the section 'Selecting RSet', if you have LEDs demanding only 20 mA, you'll only have a 170 mA typical draw.