For everybody thats need to limit the instance number of your application:

uses SysUtils, TLHelp32;
 
function AppRunningCount(const AExeName: string): integer;
var
  hSnap: THandle;
  p32: TProcessEntry32;
begin
  Result := 0;
  hSnap := CreateToolHelp32SnapShot(TH32CS_SNAPPROCESS, 0);
  p32.dwSize := Sizeof(TProcessEntry32);
  Process32First(hSnap, p32);
  repeat
    if SameText(AExeName, ExtractFileName(p32.szExeFile)) then
      inc(Result);
  until not Process32Next(hSnap, p32);
  CloseHandle(hSnap);
end;

Use:

// On DPR
// Limit to 1 instance
begin 
  if AppRunningCount(ExtractFileName(ParamStr(0))) = 1 then
  begin
  // Application codes
  end;
end.

Explaining more about this approach:
In a situation of multiples logons at the same machine the Mutex cannot provide a correctly singleton structure because the mutex handle still only under the local scope of the user. So the way to control the instance number is counting the number of times the process appear on the process list of the machine.