UPDATE (3-19-19)

You now have the choice to send a bot using a keyword "trigger" OR without the need for a user to enter a keyword.

If you set your Post Engage to NOT use keywords by toggling the following setting to "Off", then your "Non-Subscriber" message or "Existing Subscriber" message will only be sent to the user one time. This means if the user continues to enter new in a first level comments on the same Post, they will not continue to receive the same messages. This is done to prevent spamming users who are simply replying to your Post with multiple comments.

If you set your Post Engage to use a keyword "trigger", then the user will receive the "Non-Subscriber" message (if they are not yet a subscriber) or "Existing Subscriber" message each time they use the keyword in a first-level comment.
You can continue to use multiple keywords for each post. If you are using keywords, they will trigger a bot if a visitor uses the keyword in a sentence OR by itself.
For example: If I have set a keyword of 'discount' (do not use single or double quotes when setting keywords), the bot will trigger if the visitor comments either of the following:
I would like a discount, please

A word of caution if you choose to NOT use a keyword to trigger a bot to be sent. The visitor may not be expecting to receive a message from you through Messenger by simply commenting on one of your Posts. It is best practice to mention that a message will be sent so the user expectations are set properly. Giving your visitors the "heads up" may help to keep any spam reports from being sent.