The proportion of telephone lines served by manual, SxS, panel, crossbar, and ESS varied over time.
The Bell System Engineering Textbook gives a breakdown, but I don't have it handy. However, I believe in the 1970s SxS served roughly 45% of the lines, the rest shared between panel and crossbar, with ESS growing and a few manual exchanges.
The Bell System served many small towns and rural areas in addition to big cities. Before the war SxS was the way to go and continued to be into the 1970s, when the number of SxS lines peaked in 1974. Also, in cities No. 1 crossbar, developed in the late 1930s, was generally used instead of panel for new installations.
Bell System literature focuses heavily on panel and crossbar as they required sophisticated designs, but SxS was a major player. Indeed, in the 1970s Bell Labs worked on add-ons to extend the service life and functionality of SxS. Ironically, SxS could and was used for Centrex service in addition to No. 5xbar.