The system of counting followed by the Hebrews and the Semites generally was the decimal system. The highest number in the Bible described by a single word is 10,000 (Heb: ribbo', Grk: murias). The Egyptians, on the other hand, had separate words for 100,000, 1,000,000, 10,000,000. The highest numbers referred to in any way in the Bible are: "a thousand thousand" (1 Ch 22:14; 2 Ch 14:9); "thousands of thousands" (Dan 7:10; Rev 5:11); "thousands of ten thousands" (Gen 24:60); "ten thousand times ten thousand" (Dan 7:10; Rev 5:11); and twice that figure (Rev 9:16). Depending on our favorite word “context” the term may mean generation, clan, family, tribe, a company of a 1000 men, as united under one superior, or leader, a perfect (10s) period of time etc.

As far as the number of generations since Adam we do not know. There is not a complete record. Life spans back then were longer, women gave birth to larger families (the descendants of Jacob numbered 70 persons), et. al. and thus there is no way of calculating “exactly” how many generations there has been since Adam. Answers in Genesis may be a good source here, but….