Here. The story reports that AT&T has nearly 150 million customers and that the eighteen claims were filed in the last two years.
UPDATE: See comment by Gregory Gauthier below.
Arbitration, Class Actions, Unfair & Deceptive Acts & Practices (UDAP), including Discrimination
CBS News: Only 18 AT&T Customers Brought Arbitration Claims Despite Thousands of Overcharging Complaints
In a June 30 letter to Al Franken and five other U.S. Senators, AT&T said that 412 arbitration cases were commenced since January 2015, most of which were settled prior to the hearing. It’s on page 6 of https://www.franken.senate.gov/files/letter/170630ATTResponseLetter.pdf
The statistic appears to be completely wrong; I have no idea where they got the 18 figure from. My own playing around with the AAA Q1 2017 consumer arbitration dataset shows that 110 cases were filed for claims of more than $0 but at most $1,000, with the smallest amount in the data set being a claim for $19.98. Of the 110 cases, 80 were filed on or after May 16, 2015. (This is likely an underestimate, as the dataset does not include cases not closed by the end of March 2017.)