I gleaned that stat from footnote 36 of this article.  Does anyone know of the comparable figures for other major producers of law teachers like Harvard, Chicago, and Stanford?

UPDATE:  Avery Katz at Columbia Law School writes:

It sounds astounding when you say it that way [i.e., 1 out of every 12 living alumni…].  But as my colleague
Scott Hemphill and I quickly worked out using back-of-the-envelope
calculations, it’s actually pretty plausible.  Estimate the total
number of AALS faculty as 168 member schools x 50 profs/school = 8400.
Estimate the total number of Yale alums as 200/yr x 60 years – 12,000.
[Balkin and Levinson report there are about 12,000 living alums, so we
are close enough. Maybe there are 240/year when you count LLM’s.]  With
about 1000 Yale grads in teaching, this suggests that about 1 out of 8
law teachers is a Yale grad.  Sounds plausible when you look at it that
way, no?  And probably the average is higher in recent years, as the
fraction of new law teachers with Yale degrees rises.  (Larry Solum
reports that 26/148 entry-level hires last year had a Yale degree, or
about 1/5.7 — though the previous year it was only 21/153 = 1/7.3.

Using the same method of estimation, suppose the total number
of living Harvard alums are 700×60=42,000, and suppose that 1/6 law
teachers is a Harvard grad [here the trend is probably downwards]. 
This yields a fraction of 8400/(6*42,000) = 3.33% of Harvard alums a
law teacher. 

There are foreign law teachers, too, so these counts are probably a bit low.