Total current that can be drawn from a battery depends on discharge capacity (C) and ampere-hour (Ah) rating of a battery. Maximum continuous current that can be drawn from batter can be given as follows
Maximum Current (A) = discharge capacity (C) x ampere-hour(Ah)
For example, in your case if you are using 5000mAh, 25C battery, then you can draw maximum 125A continuously.
Generally discharge capacity of battery is mentioned as continuous discharge and burst discharge. Continuous discharge rating can be used to calculate maximum allowed continuous current. Burst discharge rating can be used to calculate maximum burst current that an be handled by battery. Usually burst current is can be drawn for few seconds (less than 10 seconds).
For example, in your case if you are using 5000mAh, 25C-50C battery, they you can draw 125A continuous and 250A burst current.
So for application purpose, one should always use continuous discharge rating for calculations and keep in mind burst rating only as fail-safe.
If you are using batteries in parallel then make sure all have same discharge rating. Discharge rating and voltage of resultant battery will be same as individual battery and only Ah rating will add up. So in your case, resultant battery should be 9600mAh, 25C then only it will be able to supply 240A(8x30A) continuously. In case you are using 4 batteries in parallel then each battery should be at least 2400mAh.