Calculate current load for 8 motors
Here is the thing, your motor will draw somewhere between 1.2A and 13A, depending on the load on the motor from the prop.  Odds are that the actual current will be lower than that.  Lower than the 1.2A minimum and lower than the 13A max.  So really, all we know is that the current per motor will be less than 13A.

The currents add up, so the actual current is guaranteed to be below 8 * 13A, or 104 amps MAX.  This sounds unreasonably high, and it is.  That works out to over 1,200 watts!  But thinking about that further brings up a lot of other questions that you need to consider:


What is the actual current required, given real world loads on the prop?  You cannot measure this in the lab.  It must be done in the field during extreme flight maneuvers.  Which means that to do this properly your quadcopter (I assume you have a quadcopter) must be outfitted with current sensors.  Odds are that doing this measurement is not possible, so you just have to guess.  
You said that it worked with a 5A supply.  Did it really work, or did it just spin up the props?  Just spinning the props will require a lot less current than doing interesting maneuvers.
What happens if the power supply or battery can't sustain the current?  A battery would probably just lower the output voltage, but a supply might go into overcurrent protection and/or blow up.  Lowering the output voltage would just cause a momentary power drop that you might not even notice.
The motor will not draw a constant amount of current.  Even when just idling, there are hundreds or thousands of peaks and dips in the current draw per second.  Do you care about those peaks, or only about the average (RMS) current.  Both peak and RMS current are important for different things.


Ok, so back to your question of "how do you calculate the current draw of the 8 motors?"  You need to know the actual current draw of one motor and multiply it by 8.  But as I have shown, knowing the draw of one motor is difficult. And knowing the effects of that is also difficult.  Good luck!