benchmark: changed millions and thousands to n

PR-URL: https://github.com/nodejs/node/pull/18917
Fixes: https://github.com/nodejs/node/issues/18778


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>