Is it possible to get daily Followers and Unfollower count using any API?
No. You’ll have to use the followers/list endpoint and keep a record of new ones and ones that fall off.
Can we get Organic and Prmoted impression for ads and tweets using API?
Prompted impressions may be available through the Ads API, I’m not sure as I’m not very familiar with it. Organic impressions are only available through the commercial API, GNIP.