I must make a web page that alert users , this page appair only if apache connections clients exceed specific value, for example 150 or more
As far as I know, there's no built-in, or other good way to do this. If you really must do this, this shell command will give you the number of Apache processes:
You could execute that in a Bash script via cron and then, for example, update Apache settings to show the page that alerts users if the value is above 150. Or you could execute it in PHP and show output accordingly.