Need Help with Nodemcu

I want to send "numerical value" (from 0 to 255) from a browser (PC or Mobile via app) to Nodemcu Serial monitor.

can some one please help me with the code ??

Have a look at the ESPwebserver example. Create a webpage with a html-form where you can enter a numeric value, and extract that value programatically after the form has been submitted. We are here to help, not do the job for you.

Find tutorials on how to use the ESP8266WebServer library. The methods you need are:

  const String& arg(String name) const;    // get request argument value by name
  const String& arg(int i) const;          // get request argument value by number
  const String& argName(int i) const;      // get request argument name by number
  int args() const;                        // get arguments count
  bool hasArg(const String& name) const;   // check if argument exists

This tutorial is a little bit complex but you can use some parts of it for your needs. Try to understand and adapt the handleLogin() function.