I was curious, so I looked this up: Section 5.3 of the 2010 U.S. merger-guideline explicitly classified markets with a Herfindahl–Hirschman Index below 1,500 as “unconcentrated.”

I can't quote the formula here, but HHI is, apparently, the sum of squared percentage market shares. Six equal-sized suppliers give an HHI of approximately 1,667, while seven give approximately 1,429—on opposite sides of the 2010 threshold.

Is that where you got the number?