set KEEP=5
set CUR=0

for /f %%i in ('dir /B /O-N stunnel*.log') do call :LOOP %%i
GOTO END

:LOOP
if %CUR% GEQ %KEEP% GOTO REMOVE
GOTO NEXT

:REMOVE
del %1
GOTO NEXT

:NEXT
set /A CUR+=1
echo %CUR%

:END