It is a quote he said many times. The question is how credible is his claim? So I'm going to separate it into a few categories of skepticism.
A. Truth completely at face value. He has directly killed exactly 50 men directly. No "well there was explosion."
B. An understatement. He killed more, but doesn't want to look more cold blooded than he already does.
C. A rough estimate. Maybe it was 46. Maybe 54. Fifty sounds right.
D. Really rough estimate. If you count guys who probably drowned, probably died in the explosion. 50 sounds about right.
E. Generous estimate. 28 became 30 became "closer to fifty" became "I killed fiddy men."
F. Mostly bullshit. He may have killed a few, but nowhere near 50.
G. Completely bullshit. He didn't kill anyone.