if I understand you correctly then you can use bin functionality on count(emails by whatever your fields are) or filter based equivalent to solve your 20+, 15-19, 10 - 14 etc requirement
You can also use a column selector to allow users to add various optional fields, but this may make the logic of the above requirement more complex.
Alternatively you could use a view selector in place of a column selector to keep the logic for your first requirement simple.
How is the data stored? Does it support this use in it's natural form or do you need to move it and transform it to support the requirement?
What do you mean by 'IN INSIGHT"?
Eloqua BI Insight custom report i am trying to achieve the above request.
Insight as far as I'm aware is the Eloqua reporting tool which really is OBIEE 126.96.36.199. What have you tried in order to build this analysis? What does your subject area look like? Do you have access to make modifications to the repository?