In 1922 a national broadcasting boom, consisting of hundreds of new stations, swept across the United States. But the rest of the country was merely catching up with San Francisco, California, which at the close of 1921 likely had the most stations in one location broadcasting entertainment to the public -- seven as recounted in this article.

The San Francisco-area activities reviewed below began in the spring of 1920, and were partially the result of a development which had occurred on the opposite coast. After World War One, the DeForest Company's New York City experimental station, 2XG, briefly returned to the airwaves. However, an infraction caused the station to be shut down by the local radio inspector, with little prospect for a return to the air. So 2XG's transmitter was shipped across the country, where it was issued a new experimental licence, as 6XC, the "California Theater Station". The station started daily broadcasts in April, 1920, joining over the year at least six other area stations.

For an comprehensive review of the histories of these and other early stations in the San Francisco area, see John Schneider's Radio History Archive, especially Early Broadcasting Activity in San Francisco, 1920-1925.
 
Radio News, February, 1922, page 702:

Radio  Telephone  Development  in  the  West
BY  HARRY  LUBCKE.
The development of the radiophone and the broadcasting of concerts has been taken up seriously by several firms and amateurs located on the Pacific Coast.
    At present there are seven phones working around San Francisco. The first station to come into existence was located at the California Theater and was formerly operated by the DeForest Co. It broadcasted music on a wave-length of 1,250 meters. This station started about a year ago.
    About three months after the opening of this station, a set was installed in the Presidio of San Francisco and was operated by the Signal Corps of the Army.
    For a while, these stations held a monopoly of the ether, as far as radiophones went. But their monopoly was rudely broken by the installation of a station at the Fairmont Hotel in San Francisco, operated by the Leo J. Meyberg Co., dealers in radio goods. At first this station used only one 50-watt tube and it sure did push the electrons into the air. It was the delight of the "hams" with crystal detectors because it used to come in loud enough to show off to visitors.
    Then in quick succession phones sprang up in Oakland, an adjacent city to San Francisco; at the Hotel Oakland, operated by the Western Radio School; the station at Los Altos, operated by the Colin B. Kennedy Co.; the Radio Shop of San Jose, and another in Oakland operated by the Warner Brothers, dealers in that city.
    Until very recently concerts and press news were broadcasted on various wavelengths, and all one had to do if he did not like the record one station played was to give the variometers a twist and another station would come in.
    But all this has stopped. A new government regulation compelling all phones and C.W. stations to operate on a wavelength of 360 meters has been passed. Then all the radiophone men assembled and had a pow-wow about the schedules. So they finally fixed it so they would not interfere with each other.
    Now one can receive press, weather, market reports and concerts in the afternoon from 4:30 to 5:30 P.M. and at night from 7:10 to 9:00 P.M. During this time several stations would be operating in turns.
    The old California Theater station was taken over by the Atlantic-Pacific Radio Co. which is going to remodel the set to work on 350 meters.
    Several months ago we had quite a treat from the Fairmont Hotel in the form of a concert rendered by members of the Scotti Grand Opera Co., which was giving performances in San Francisco at that time.
    Also, at frequent intervals, lectures are broadcasted from the Presidio. They are on radio topics of general interest and are very instructive.
    It is very interesting to note that the increased efficiency of the stations as time progressed. At first the California Theater used nearly one kilowatt, and obtained about 4 amperes radiation. Later the Fairmont Hotel, which uses a great deal less power and secures only a little less than ampere radiation, comes in from two to three times louder.

With the exception of the Presidio station, AGI, which was a government station that also was sometimes reported using the callsign of 6XW, all of the above stations initially operated under Experimental licences. The adoption of the December 1, 1921 regulations requiring entertainment broadcasting by non-government stations to be conducted under Limited Commercial licences on 360 meters (833 kilohertz) meant that these stations had to get the new licences, with a resulting callsign change. The table below is a general overview of the six non-government stations reviewed in this article, based on Radio Service Bulletin and licence information:

Initial Experimental AuthorizationBroadcast Service Authorization
CallOwnerLocation1st RSB EntryCallOwnerLocationDates
6XCLee De Forest (Inc) [California Theater]S. F.10/1/1920KZYAtlantic-Pacific Radio Supplies Co.Oakland12/9/1921-1/24/1923
6XGLeo J. Meyberg Co. [Fairmont Hotel]S. F.6/1/1921KDNLeo J. Meyberg Co.S. F.12/8/1921-5/1/1923
6XAJPreston D. Allen [Hotel Oakland]Oakland8/1/1921KZMPreston D. AllenOakland12/9/1921-6/23/1931
6XACColin B. Kennedy Co.S. F.4/1/1921KLPColon B. Kennedy Co.Los Altos1/2/1922-3/9/1923
6XAGThe Radio ShopSunnyvale7/1/1921KJJThe Radio ShopSunnyvale12/20/1921-6/16/1923
6XAMStafford A. WarnerOakland10/1/1921KLSWarner Bros.Oakland3/10/1922+ (now KMKY-1310)