And yes, he apparently was carrying three channels from Philadelphia (3,6, and 10) on-channel and without downconverting 10 to a low-band channel.
Also during 1948, two other CATV systems were started, in Astoria, Oregon and Tuckerman, Arkansas. Each carried one channel, and each started with very few actual customers. Civic boosters in both communities claim to have been "first".
Ed Parsons, owner of the Astoria system, claims that he began operations on Thanksgiving Day 1948 carrying KRSC (now KING) Seattle, 125 miles away. An article in the Fall, 1996 issue of "Invention & Technology" supports this claim, but I suspect that it was based on secondary sources. More about this article in the following T-D posts:Jimmy Davidson, owner of the Tuckerman system, apparently didn't keep any records. However, a story in the November 13, 1948 edition of local newspaper states that "television programming" was received here on that date.
I have always believed that Astoria was first, but Walson makes a good case for being first. So depending on which system came first, the answer to the original question is either one or three.
Back in the late 140s, that was almost universally true. Walson, Parsons, and Tuckerman all owned hardware, appliance, or furniture stores. In a story repeated numerous times in small communities across the United States, a store owner gets wind of this new thing called "CATV" and decided to build a CATV in his community so he can sell TV sets.
The idea worked: he started selling lots of TV sets. And so did his competitors: within a few months, every hardware store, appliance store, furniture store, gas station, grocery store, and (I once heard) even a funeral home, got into the TV-set business.
Within a year, the community was saturated and TV-set sales plummeted. But the original entrepreneur discovered that he was making more money selling CATV service than he was making from selling TV sets, so he sold the store and went into CATV full time.
And thus an industry was born.
References: The Hauser Oral and Video History Collection: John Walson. Denver: The Cable Center, August 27, 1987.  Mary Alice Mayer Phillips. CATV: A History of Community Antenna Television. Evanston: Northwestern University Press, 1972.  Archer S. Taylor. Pioneer Tales from Cable TV History. Denver: The Cable Center, 2005.  George Mannes. "The Birth of Cable TV." American Heritage of Invention & Technology. Rockville: American Heritage Publishing Company, Fall 2006.  "First Television Here on Saturday, November 13, 1958." Tuckerman: The Tuckerman Record, November 13, 1958. Quoted in Taylor, p. 4.