Figures from Billboard, June 3 1967, show that in 1965 over two-thirds of LPs sold were still in mono.
Two further things should be noted about these figures:
1. They are for LPs in all categories of music, including jazz and classical; if popular music LPs had been separated out the percentage of mono sales would undoubtedly have been substantially higher.
2. The figures are for "Non-Rack Only", in other words for sales through non-discounting record retailers. They exclude "rack" sales made through discount stores and record racks in supermarkets, petrol stations and so on. The rack outlets, selling to the cost-conscious, would mainly have been stocking mono records, bearing in mind that at this time stereo LPs carried a substantial $1 premium at list price. In 1965, rack sales accounted for 37% of all record sales (Billboard, Jun 4 1966)
All this changed dramatically in the second half of 1967, after the record companies raised the price of mono LPs to the same level as stereo. Given the choice at the same price, most customers started opting for stereo. This change also followed a publicity campaign by some of the big companies to convince customers that they would have no problem playing stereo records on mono equipment as long as their pickup cartridge had been bought in the last few years. The industry – manufacturers, distributors and retailers – couldn't wait to simplify their lives by getting rid of mono, and in the US most of the record companies stopped producing mono LPs altogether during 1968. The UK lagged behind, to the extent that Nashville Skyline (April 1969) still received a mono release; but CBS UK had to produce it themselves as a fold-down from the stereo tapes, and it only sold in small numbers.