Currently, the number of parameters that can be set for External Database Connector is fixed, even when used in the In clause.
select * from customers where country in ({Parameter1}, {Parameter2}, {Parameter3});
When using the External Database Connector, I want you to create a string variable called Parameter1 in your microflow and then be able to loop through a list of values (Parameter1 = 'Germany, France, UK, ・・・') and set the value for that single parameter.
select * from customers where country in ({Parameter1});
This allows you to specify search conditions dynamically, making it easier to implement search actions using SQL.
In newer versions of the Database module it may be possible with the Parameter entity. May be worth checking.
https://docs.mendix.com/appstore/modules/database-connector/#parameterlist