public class RelatedContacts
{
    public Static List<Contact> Relatedcontacts = new List<Contact>();
    public static integer count=0;
    public Static List<Lead> OtherLeads = new List<Lead>();
    public static void LeadConvertion(Map<Id,Id> RelatedLeads)
    {
        List<Database.LeadConvert> converts = new List<Database.LeadConvert>();
        String convertedStatus = [SELECT MasterLabel FROM LeadStatus WHERE IsConverted = TRUE LIMIT 1].MasterLabel;
            for(Lead Ls:[Select id,Parent_Lead__c from lead where Parent_Lead__c=:RelatedLeads.keyset()])
            {
                If(Ls.Parent_Lead__c <> null && count <20)
                {
                    Database.LeadConvert lc = new Database.LeadConvert();
                    lc.setLeadId(Ls.Id);
                    lc.setConvertedStatus(convertedStatus);
                    lc.setAccountId(RelatedLeads.get(Ls.Parent_Lead__c));
                    lc.setDoNotCreateOpportunity(true);
                    converts.add(lc);
                    count =count+1;
                }
                if(Ls.Parent_Lead__c <> null && count >=20)
                {
                    //Lead lss=new lead();
                    Lead ll=new Lead(Id = Ls.Id);
                    //ll.id=Ls.id;
                    Count=Count+1;
                    //ll.Status='Not Visible';
                    ll.Parent_Lead__c='';
                    OtherLeads.add(ll);
                    //OtherLeads.add(lss);
                }
            }
        If(!OtherLeads.isEmpty())
        {
            Update OtherLeads;
        }
        Database.convertLead(converts, false);
    }
}