Text Filed Filter

Then, what is wrong with the drop down field?

users want to enter the value manually. they are not accepting the dropdown. 

But entering a value manually while selecting a value from a predefined list ... does not work together in my mind.

For me, manually entering a value equals search, selecting from a predefined list equals filtering.

Alternatively, if you still want to have 20 different text fields for users to type and filter, you can create custom filters using text fields. Whatever value the user types can be stored in a variable. Then, create a rule to query the record data to display in a grid, and parameterize the query filter to filter the data by the typed values. However, I would  recommend using the default search option as it already accomplishes what you're attempting to build. 

you can check here : Manually creating a filter on the grid