Three ways of expressing it:
+1 (relative charge)
Or +e , where e is the elementary charge.
Potassium atoms lose one electron when they form an ion, so it has one more proton than electron giving it an overall positive charge equal to the charge on a proton. Protons and electrons have equal charges but opposite sign.
So for simplicity we can say that potassium forms a +1 ion, indicating that it has 1 more proton than the total number of electrons.
In atomic terms charges only change in multiples of the elementary charge. The elementary charge, e is the magnitude of the charge on an electron or proton. Atoms can only lose or gain whole numbers of electrons so the overal charge can be expressed in terms of e. For a potassium ion it is +e.
The value of e in coulombs is
All elements in group 1 of the periodic table form a positive ion with the same magnitude of charge as potassium.