According to page 3 of the board documentation, each of the 12 output terminals is taken directly from the output of an optoisolator (which is the collector of an NPN transistor - the emitter of that transistor is grounded), and there is a pull-up resistor from that point to the +5v isolated power supply. I do not know the value of the pull-up resistor, however I believe that the resistor will limit the maximum current that you can source from each output terminal. From the photograph, it appears that the pull-up resistors are inside of a couple of SIP or DIP resistor networks. So, it would not surprise me to find that those were low-power 1K or 4.7K resistors. I would expect the maximum current that you could source from any output terminal would be very small. Also, I would not apply more than +5v to any output terminal.
The maximum current that you can sink through each output terminal depends on the rating of the optoisolator.
If you have that board, can you look for the part number of the optoisolators? Maybe you can find a spec sheet for the optoisolator that will tell you the maximum voltage and current that it can handle. If you can read the part number or value for the pull-up resistor networks, then that will also tell you how much current you could source from each output terminal.
That one does not look like a well-designed, high-quality board, and I would be concerned about using such a board in my system.