That's the number of seats available for sale in that booking class. Generally speaking, going further right gets more and more restrictive (e.g., Y, B, M, V, T or something like that).

Less restrictive seats already include the more restrictive seats, so Y5 V2 doesn't mean 7 seats, it means 5 expensive seats or 2 cheap seats (i.e., 2 cheap and 3 expensive, 1 cheap and 4 expensive, or 0 cheap and 5 expensive seats can be sold by NW or travel agents).