He doesn't even need the 500 liters of energy water. What he needs is 90 liters. But he has his reasons for asking for 500 liters instead of 90.
90 liters is all it will take him to reach the peak of rank 1 in terms of spiritual power. With 20 units of spiritual power, he can begin to ascend to rank 2.
After reaching rank 2, he will need 200 liters of energy water to gain another 20 spiritual power to reach the peak of rank 2. It will be handy if he can have that 200 liters now and take it as soon as he reaches rank 2.
So in summary, he needs 90 liters and wants 200 liters for a total of 290 liters of energy water. The extra 210 liters is insurance for his future needs. Even if he loses the five of them, he will still have the 210 liters.
The extra 210 liters is also to throw off their calculations about his power. By asking them for 500 liters, they will believe that he is at least at rank 3 and probably at rank 4 to need that much.
