The number of independant bus operators in the Lothians does seem to be ever decreasing, what with Eve now bought by Lothian only months after Horsburgh stopped operating public bus services. While many companies operate routes into the Lothians, I believe there are only 4 independent operators based in the area who still run public services; SD Travel, Prentice Westwood, Edinburgh Coach Lines and Prenctice of Haddington. (I'm not counting Waverley Travel, the RBS shuttle isn't a public service).
It does seem like the trend is going to contiue, and currently only Prentice of Haddington and SD Travel have significant networks of bus routes.
Can't be sure on the exact number, but even 10 years ago there were quite a few more independants in the area.