set /a countfiles-=%countfiles%
This will set countfiles to 0. I think you want to decrease it by 1, so use this instead:
set /a countfiles-=1
I’m not sure if the for loop will work, better try something like this:
:loop
cscript /nologo c:\deletefile.vbs %BACKUPDIR%
set /a countfiles-=1
if %countfiles% GTR 21 goto loop