in the code below the dp.CS_LocationsCSV__c is comma selerated string of account numbers  ie  123,145,16,143  and i need to use them in the soql querey  SELECT Account__r.id ,source_key__c  FROM ERP_Data__c where source_key__c = :csv   this works fine with one account number but i may have up to 50, how can i get them into a useable set for the querey?

trigger CS_links on Distributor_Program__c (after update) {

   if(checkRecursive.runOnce())
    {

// get a list of all Distributor_Program__c to evaluate

Set<String> csv = new Set<String>();
Set<Id> changedcsv = new Set<Id>();

for (Distributor_Program__c  dp :trigger.new){

    String oldcsv = Trigger.oldMap.get(dp.Id).CS_LocationsCSV__c;
    String newcsv = dp.CS_LocationsCSV__c;

    if (oldcsv != newcsv){
    changedcsv.add(dp.id);
    csv.add(dp.CS_LocationsCSV__c);
    }

}
// delete all exsiting Participating_Branch_Locations__c if CS_LocationsCSV__c has changed

list <Participating_Branch_Locations__c> pb = [select id from Participating_Branch_Locations__c where Distributor_Program__c in :changedcsv];
delete pb;

// find the accounts to link to the Distributor_Program__c where the CS_LocationsCSV__c has changed

map<string , account> accids = new map<string , account> ();

list<ERP_Data__c> erps = [SELECT Account__r.id ,source_key__c  FROM ERP_Data__c where source_key__c = :csv];  // todo use the csv string 

for (ERP_Data__c erp :erps){
    accids.put(erp.source_key__c  , erp.Account__r );
     }

//create the Participating_Branch_Locations__c and link to the account and Distributor_Program__c 


list<Participating_Branch_Locations__c> npb = new list<Participating_Branch_Locations__c> ();
        for (Distributor_Program__c  dp :trigger.new){

        account a = accids.get(dp.CS_LocationsCSV__c);
            if (a != null){
            
           
                for (erp_data__c acc : erps) {
                Participating_Branch_Locations__c apbl = new Participating_Branch_Locations__c();
                apbl.Account__c = a.id;
                apbl.Distributor_Program__c = dp.id;
                apbl.key_id__c = a.id + '' +  dp.id ;
                npb.add(apbl);
                }

            }
        }
        insert npb;
        }
}