type TApiGetHTTP = class(TTharead)

....

var
 HTTP : array of TApiGetHTTP;

var
  ContentLength: Int64;
  Offset, Chunk : Int64;
  RangeBegin, RangeEnd : Int64;
  PartCount : Cardinal;
  I : Integer;
begin
  ContentLength := GetUrlFileSize(AURL);
  PartCount := 5;
  
  Chunk := ContentLength div PartCount;
  Offset := 0;
  
  For I := 0 To PartCount -1 do
  begin
     SetLength(http, i+1);
     RangeBegin := Offset;
     RangeEnd := Offset + Chunk;
     Offset := Offset + Chunk +1;
     HTTP[I] := TApiGetHTTP.Create(True);
     HTTP[i].Suspend;
     HTTP[i].RangeBegin := IntToStr(RangeBegin);
     HTTP[i].RangeEnd := IntToStr(RangeEnd);
  end;

  For I := 0 To Length(http) -1 do HTTP[i].Resume;