This tool can be used to see how many messages you've sent to other people using JSON formatted Facebook Messenger data that you can download from Facebook (click here to find out how to do that)
Once you've downloaded the data you can use the file picker in this program to select a JSON file containing your Messenger data. Alternatively, if you just want to test the program you can simply download the test JSON file from the json folder in the GitHub repository here (right click the link and select save file).
You can download and run this tool locally in your browser, or just click here.