I initially thought the frequency was the number of corps, such that 144 corps had 0 deaths over the 20 year period, 91 corps had one death and so on. However, there are 25,000 - 50,000 men in a corps, so if the sum of the frequencies were the number of corps, that would equate to 14,000,000 men, which seems unrealistic.
I'm having real trouble interpreting the question. If anyone could provide guidance, it would be much appreciated!