// records to be updated received from json
jsonBody = '[{"count__c":"445", "downloads__c":"340"}, {"count__c":"440", "downloads__c":"240"}]';

List<Data__c> dList = (List<Data__c>) System.JSON.deserialize(jsonBody, List<Data__c>.class);

countList has unique count__c values, say: 445,440 // to use in the IN clause.

// Querry parent for those plan ids in daily data json
List<Parent__c> parentList = [SELECT Id, Name FROM Parent__c 
    WHERE count__c IN :countList];

List<Data__c> dataToInsert = new List<Data__c>();

// Loop through dList - inner loop
for(Data__c dRecords : dList) {
     for(Parent__c parentRecords : parentList) {         
          if(dRecords.count__c  == parentRecords.count__c) {
                dRecords.downloads__c  = parentRecords.downloads__c ;
                dataToInsert.add(dRecords );
           }
      } 
} 

List srList = Database.insert(dataToInsert, false);
for(Integer i=0;i<srList.size();i++){
    if (srList.get(i).isSuccess()){
        srList.get(i).getId();

    }else if (!srList.get(i).isSuccess()){
        // DML operation failed
        Database.Error error = srList.get(i).getErrors().get(0);
        String failedDML = error.getMessage();
        recsToInsert.get(i);//failed record from the list
        system.debug('Failed Id: '+recsToInsert.get(i).Your_field__c);
    }
}

@isTest
private class Connect_Test {
    @isTest static void testCall() {
        Profile p = [SELECT Id FROM Profile WHERE Name='System Administrator' LIMIT 1]; 
        user intUser = new User(Alias = 'standt', Email='standarduser@testorg.com', 
            EmailEncodingKey='UTF-8', LastName='Integration User', LanguageLocaleKey='en_US', 
            LocaleSidKey='en_US', ProfileId = p.Id, 
            TimeZoneSidKey='America/Los_Angeles', UserName='intuser@co.com');
        insert intUser;

        System.runAs(intUser) {
            List<Parent__c> ms = new List<Parent__c>();
            Parent__c msRec1 = new Parent__c(count__c = '445');
            Parent__c msRec2 = new Parent__c(count__c = '440');
            ms.add(msRec1);
            ms.add(msRec2);
            insert ms;

            String strIds = '445, 440';
            list<String> countList = strIds.split(',');

            List<Parent__c> parentList = [SELECT Id, Name FROM Parent__c 
                WHERE count__c IN :countList];

            List<Data__c> recsToInsert = new List<Data__c>();
            Data__c ddRecs1 = new Data__c(downloads__c = 'a1IO001110AakH2MAJ');
            Data__c ddRecs2 = new Data__c(downloads__c = null);
            recsToInsert.add(ddRecs1);
            recsToInsert.add(ddRecs2);
            try {
                List<Database.SaveResult> srList = Database.insert(recsToInsert, false);
            } catch(DMLException e) {
                throw new DMLException('Unable to Perform the DML Operation: ' +e.getMessage());
            }
            Test.startTest();
            Test.setMock(HttpCalloutMock.class, new MockHttpResponse());
            Connect.call();
            Test.stopTest();
        }
    }
}