K+ is the symbol for a potassium ion with a positive charge. The positive charge means that the ion has lost one electron.
The neutral atom of potassium (K) has 19 electrons because its atomic number is 19. When a potassium atom loses one electron to form a K+ ion, it becomes positively charged. The loss of the electron leaves the K+ ion with 18 electrons.
So, the K+ ion has 18 electrons.