thats one every 3 months for 20 years (that he is admitting to)
If you check out his Wikipedia entry, in the box on the right-hand side of the screen, it says;
Convicted of 49, confessed to at least 71, presumed to be 90+
That ‘presumption’ comes from law enforcement sources. Ridgway himself said he pretty much lost count of how many he’d killed.
http://en.wikipedia.org/wiki/Gary_Ridgway