I have two custom fields Chennai_DC_No__c,Bangalore_DC_No__c and Location__c (picklist) contains Bangalore,Chennai

One field is incrementing and another one is not incrementing , every time it display 1.

trigger ChennaiNos on Demo__c (before insert) {
List<Demo__c> OpList = new List<Demo__c>(); 
    OpList=[SELECT Chennai_DC_No__c FROM Demo__c]; 
    for(Demo__c c : Trigger.new){ 
    if(OpList.size() > 0){
                if(c.Location__c!= 'Bangalore'){ 
                  if(OpList[0].Chennai_DC_No__c!= null){                  
                    c.Chennai_DC_No__c =OpList[0].Chennai_DC_No__c +1;
                    } 
                    else{
                    c.Chennai_DC_No__c =0+1;
                    }
                    }
                    }
                    if(OpList.size() > 0){
                if(c.Location__c== 'Bangalore'){ 
                  if(OpList[0].Bangalore_DC_No__c!= null){                  
                    c.Bangalore_DC_No__c =OpList[0].Bangalore_DC_No__c +1;
                    } 
                    else{
                    c.Bangalore_DC_No__c =0+1;
                    }
                    }
                    }
                }

}