I don't know the percentage of '64 convertibles that came with bucket seats but from one of Jason's trivia questions a while back, only 2.8% of all '63 Cadillacs came with buckets. I would guess the numbers were similar for '64s but don't know how it breaks down according to body style.
Well at least someone replied LOL. Based on those numbers it would be about 3091 for 1964 since there were about 110,000 produced. I saw a some verbiage on a Mecum auction car of stating around 1800 convertibles had buckets so maybe that number is correct.