Can i get email notification each time error log added/increase?So faster to response

Probably not in Apache itself.

However, you could set a cron job that executes every 1-5 minutes and calls a script that checks to see if error_log has changed and send an email if it has.

Not sure I'd want this because if a bad bot hits your site testing vulnerabilities it can easily clutter up error_log with errors that don't really exist and send a lot of worthless email in a hurry.

can anyone give me the cron job command example for this please

no problem with a lot of emails, we can disable and enable the cron anytime temporary if needed

You can use the ErrorDocument directive to redirect error responses to a script that generates an error page and also sends the admin an E-mail. E.g.:

ErrorDocument 500 /email-error.php