Googling it doesn’t work, but this THT article quotes the numbers:

K/PA: 150 BF
GB%: 150 BF
LD%: 150 BF
FB%: 200 BF
GB/FB: 200 BF
K/BB: 500 BF
IF FB%: 500 BF
BB/PA: 550 BF
BABIP: Doesn’t reach a 0.50 r-squared at 650 or below.
HR/FB: Doesn’t reach a 0.50 r-squared at 650 or below.

(FYI, r-squared of .5 is the same as an r of .70.)

Just to be clear, we’re not saying that stats all of a sudden become reliable at those BF totals. Pizza picked a somewhat arbitrary r-squared cutoff and found how many BFs were necessary to achieve that level of correlation. What this list is really good for is to show the relative significance of different stats and how soon the data stabilizes, relatively. We’re always regressing, even with five seasons worth of data, it’s just that you regress less the more data you have, and you regress less for different stats given the same amount of data. /soap box