Well firstly you shouldn't be taking into account the atomic number; lepton number is conserved.
Potassium has 0 lepton number. The electron has a lepton number of +1.
Argon has 0 lepton number number. The electron neutrino has a lepton number +1
0 + 1 --> 0 + 1
1=1
Lepton number conserved.
Your thinking is incorrect because in electron capture, a proton decays into a neutrino by capturing an inner shell electron (it's in the name!). Therefore, both the proton number and the number of electrons in the atom decrease by 1.
EDIT: so it's actually 18 + 1 --> 18 + 1
19=19 hence lepton number is conserved.