We have to loop through a database table and get all the attachments from the db

You could do it in batches based on the primary key or some other criteria.

In regards to the number of rows returned by the node, this is the setting in charge:

## The limit to the number of rows returned in the Database Access node
#resources.appian.process.application.arv.limit=10

When u have more no of rows in a table, other way around is:
you can configure the pagingInfo parameter(default parameter) of your query rule.
Say u have query rule rule!myQueryRule(pagingInfo). you can configure this within a with loop in a script task and do your batch processing within that.

@phanibabuk As said by Eduardo, retrieving in batches is the only solution. I would like to recommend the usage of a!queryEntity() incase if your implementation involves Appian's OOTB query rules other than Query Database node (Reference: https://forum.appian.com/suite/help/7.10/Querying_Data_From_an_RDBMS.html).