That's the number of times you've died thus far.
That's the number of flowers that appeared on your head, which were slowly taking over your vision.
The first time you died was from Toriel's fireballs as they burned through your stomach, and a look of both insanity and horror crossing her features as your soul shattered into tiny pieces.
When you woke up after resetting, you found a yellow buttercup stuck to your forehead. But you left it be since plucking it out was incredibly painful. Even Flowey, the timid flower you befriended, couldn't explain to you why they were appearing on you every time you reset.
The second flower appeared on your left cheek after you met Sans-a sharp-toothed, snarky skeleton with a knack for puns....or at least you learned that after he electrocuted you to death with his joy buzzer.
But nonetheless, you pretended the incident never happened and befriended Sans.
The 18 other flowers were from your battles with other Monsters, especially Undyn