This is a great project. I say after it's finished, you make all kinds of crazy lists like alleged kills, such as the 77 other Tudor mercs Guts was supposed to have killed in the 100 man fight.
Also, shouldn't we be taking bets on this? I'd call about 400 right based on the numbers thus far (my gut instinct, har har, says 500 though), except I think Guts will start to slow down in later volumes (not in a war, less human killing, more big fish and Apostle hunting), unless you're counting all of Roshinu's horde. The numbers could skyrocket! He basically committed genocide. The burning barn alone should be a blast to count, and I just reviewed some pages of Guts swinging into a giant swarm of them covering the DS with bug squishy on a few occasions. Good luck, BTW. =)
P.S. I call 500. =)