If you tap just once you will be charged € 1.50, the amount for a 100-minute BIT ticket (valid for 100 minutes from the first tap, while on the metro only for a single journey. This means that your journey on the metro can include different lines, but without passing through the turnstiles if you change between lines A-B-B1; passing the turnstiles is permitted only to change between line A and C.)
If you purchase more than four 100-minute BITs within 24 hours, at the fifth BIT purchase, the "best fare” feature will be activated and you will be charged a 24-hour Roma ticket for € 7, which is cheaper than five or more BIT tickets. The 24 hours of validity of the ticket will be calculated starting from the time of the first tap.
Remember, to apply the "best fare" feature all your taps must be made with the same device. For example, if you start your journey with your contactless payment card, you must continue with the same card and you cannot use other NFC devices (smartphones or smartwatches), even if you have virtualized the same physical card on them. Same thing if your first tap is done with a smartphone or smartwatch.