Seed Count
For Normal and Region modes, seed count is the number of places the biome will be placed. Each of those seeded areas will grow out somewhat, depending on the Growth setting.
Depending on your specifications and the amount of exposed rock, the program may not be able to find enough areas to seed. It will try a certain number of times, but give up so that the program doesn't get stuck in an infinite loop. The log will show how many areas were seeded, something like this:
You can try again, and often you will succeed a few more times. Or you modify things like the maximum slope or height settings.
Seed Count is ignored when using Paint mode.