The result is shown in the status bar of the Find window.

This can be macro-recorded for easy running; the downside is that to see the result, the Find window has to be open.

Running such a script is something I’ve never done

We have a FAQ on that

Simple Pythonscript, called WordCountInSelectedText.py:

# -*- coding: utf-8 -*-
from __future__ import print_function

from Npp import *
import inspect
import os

#-------------------------------------------------------------------------------

class WCIST(object):

    def __init__(self):
        self.this_script_name = inspect.getframeinfo(inspect.currentframe()).filename.split(os.sep)[-1].rsplit('.', 1)[0]
        self.total = 0
        def match_found(m): self.total += 1
        editor.research(r'\w+', match_found, 0, editor.getSelectionStart(), editor.getSelectionEnd())
        self.mb('Words in selected text = {}'.format(self.total))

    def mb(self, msg, flags=0, title=''):  # a message-box function
        return notepad.messageBox(msg, title if title else self.this_script_name, flags)

#-------------------------------------------------------------------------------

if __name__ == '__main__': WCIST()

See this FAQ