It's not unreasonable to think that in his younger years, the amount he drank wound him up hemorrhaging and in the hospital in 1954, as in the well-documented (if not possibly somewhat embellished for the sake of literature) stint from Life and Death in the Charity Ward. We know he embellished some things, at least that's the most reasonable conclusion. But there are examples of him explaining how much he drank. An inscription to Arthur Appelbaum, his attorney, indicates that he wrote Post Office in 21 days on a pint a night. Are we to take him at his word here? Maybe. In later works, he discusses drinking about two bottles of wine a night. At 1,500 mL and 12% alcohol (a typical percentage from the 70s and 80; they tend to be higher these days), that's 180 mL of ethanol. That's the equivalent of a 375-mL "pint" of 48% Laphroaig Quarter Cask. That'll mess you up for a few hours, and it ain't great for the long-term, but it's not going on a complete bender.
But one doesn't need to be what one claims to be effective. Does one need to be completely off their rocker to write an effective book about a psyche ward? I think Ken Kesey did a fair job. And if Bukowski drank half or less of what he claimed, does it lessen the quality of the output? Must one be exactly what one professes when writing fiction?