Replace String with increasing numer [SOLVED]

[AlphaInc]

Hello everybody,

I have a text-file with multiple strings which I want to be replaced by an increasing number.
For example there is there the string "example" 7 times in it and I want to replace "example" with a number:

Before:
This example is an example on how to make an example with the word example. the example ends here with an example. for example.

After
This 1 is an 2 on how to make an 3 with the word 4. the 5 ends here with an 6. for 7.

How do you do that?
Also, while developing the script, I want it to look for every text-file inside my folder not just a specified file (see code below).

This is what I have now but it doens't work:

#!/usr/bin/env python3

fileCounter = 1

with open('*.txt', 'r') as file :
  filedata = file.read()

filedata = filedata.replace('example', ''+fileCounter)

with open('*.txt', 'w') as file:
  file.write(filedata)