If the yellowstone eruption, the earth will no sunshine for 6 to 10 years due to the ash from eruption?

Indeed.

Or there will just be a small disastor, killing a small amount of people, and ash is sent around a few states.

Or somewhere in between...
The Yellowstone caldera is overdue for a super eruption if you go by the last 2 big ones (640,000 and 1.3 million yrs ago), but it rises and falls. It's gone up a lot in the last 100 years, but it most recently has decided to slow down and even stop... not that that really assures that it won't erupt anyway.
Basically no one really knows.