What is the best way to achive: I have a form with around 10(country, ssn,

One option would be to have a "length(trim(filternulls({pv!one,..., pv!ten}))) = 1" decision straight after the form and direct back if false. Then you could display an error flag 'Please enter only one search criteria'. You can use the same node to direct the flow depending on the entered value.

I m sorry in my earlier post. I meant one input is anyway mandatory and 9 are optional, which said minimum one input and max 10 inputs.
And also, the result should be displayed in a paging grid and in the paging grid user can select multiple rows and the selected rows to be displayed in another grid.
The display part in grid and data handling can be managed but my concern is how to get the data onto the first grid. I suppose I can only use query rules to get the data onto the paging grid. but then how to have the dynamic where clauses in Query Rules??

Appologies, on your first point, a decision "length(trim(filternulls({pv!one,..., pv!ten}))) > 1" should do the trick- or and(isnull(pv!one), ... isnull(pv!ten)) or something.
On your second point, if you can't use query rules to do exactly what you need you could create a view in the DB (restrict your tables to the required columns) and use a query db node to do a direct SQL query. This would give you much more latitude.

Regarding making a query rule:
As Iain suggests, making a view on those 10 fields can help with making the search query rule. Also, the "includes" operator can handle null rule inputs in query rules. So if each field's query condition uses the "includes" operator, you won't have to filter out your null values for the query rule to execute. For more info:
forum.appian.com/.../Rules

Thanks Iain and Bishnu.. will check with "includes" operator.. Hope it helps...