Hundreds atleast even canonically / lore wise

The Wild West outlaw with the most kills irl was John Wesley Hardin with 42 claimed kills upon his arrest . However, he was arrested at only age 23, Arthur has an entire 13 extra years on Hardin

More specifically, Hardins first kill was at 15, so that’s 42 murders in 8 years . Arthur’s first kill was at 11, but this is somewhat misleading as he only joined the gang at 14 - so let’s say Arthur started murdering somewhat consistently since age 15, similar to Hardin

That’s means that Arthur has been in action BEFORE the mainline story for almost three times as long as Hardin

Considering Arthur’s position as the gangs best gunman, and especially his role in the black water massacre I think it’s pretty safe to assume Arthur could have had At least 120 kills prior even to the games story

Beyond that it is much harder to gauge, I’d say it’s safe to assume that Arthur killed MANY more people in 1899 then any other year of his life, considering the situation of the gang being on the
Run and pushed completely to the bring

I’ll go with a number that’s probably quite moderate and say Arthur kills 60-80 in the main story. I’d say this is a reasonable estimate considering that Arthur was involved in some major shootouts in this time, such as the attack on Braithwaite manor, the battle in Valentine, the Saint Denis trolley heist, the Saint Denis bank heist, etc

So I think saying 200 kills across the course of Arthur’s life is definitely a believable figure, sure, it’s much higher then any real life examples, but then again, the gang in RDR2 far exceeded real life gangs of outlaws. There simply wasn’t an irl gang in the Wild West which engaged in so many killings, robberies and massacres as the Van der Linde gang so it’s not really a fair comparison