How to show how many people have signed up on my website?

I have an HTML form that asks users for their emails, and a .txt file where the emails are stored line by line. How do I show on my website, a "counter", like so:

"Join x number of people who already signed up!"

Where x is an int that counts the number of emails in the .txt file? I am using flask for my web server, combined with HTML, CSS, and Javascript.