Carter Family:
Sara Carter, vocal, autoharp; A. P. Carter, vocal; Maybelle Carter, vocal, guitar.
Recorded May 9, 1928, Victor Studio, Trinity Baptist Church, 114N. 5th St., Camden, NJ.
You are in luck, as the Victor archive actually preserved the sales numbers on this record. As fixbutte below has already mentioned, this particular record sold 141,981 copies between 1928 and 1936. Such info is rare, but if it exists, it is usually listed on the record's DAHR page.
Re Victor release dates in the Discography of American Historical Recordings (DAHR):
According to my own research, the release dates there are mostly the dates of the monthly Victor catalogs for national US releases. As seen in the "Advance Records Bulletins" in the Talking Machine World magazine (explicitly as of Sep 1923), these catalog dates don't indicate the actual release days for Victor records which were nearly always on Fridays of the previous month.
Additionally, there were also Southern series catalogs of Victor records, as now explicitly stated by DAHR for Victor 21433. These regional releases might have much later national release dates. For example, the mentioned Jimmie Rodgers record with Southern release date "8/3/1928" was listed in Talking Machine World of October 1928, p. 124, "for October 26", apparently meant for the national release.
As a result, I've corrected the release date for this record here to August 3, 1928, like the neighboring Jimmie Rodgers record. Praguefrank's date "08-03-28" for this one, though he incorrectly reads it as 8 March 1928, seems to confirm the correction. In contrast, the "5/1929" release date in DAHR, possibly coming from the first appearance in a national Victor catalog, is implausible for the first release of this record.
It doesn't help though that Joel Whitburn, in his Pop Memories 1890-1954 book (1986), lists the "Keep On The Sunny Side" side as a national #9 hit as of November 1928, because his imaginary charts are even less credible, see my comment on Victor V-40000.
This was the fourth record of the Carter Family, and the first one from their second session, not in Bristol, TN, anymore but at the Victor studio in Camden, NJ. It was a big folk hit (country & western wasn't the genre name then), especially the hopeful "Keep On The Sunny Side", strongly sung by Sara Carter, with her husband A. P. and her cousin Maybelle providing harmony.
There is uncertain information about the release date though. The date given on the Discography of American Historical Recordings (5/1929) is implausibly late, out of the chronological order of the Victor catalogue* and about one year after the recording, but the release dates on that site have to be treated with caution anyway. However, the number of sold copies from the original Victor documents listed there, although not to be considered authoritative by definition, is considerably high (141,981) and gives a good impression of the record's success.
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* The second previous number (Victor 21432) is listed as released in 7/1928, and the previous one (Victor 21433), a Jimmie Rodgers record, is listed with an accurate release date of 8/3/1928 (which was adopted by Praguefrank for this record here but apparently misread as 8 March 1928).