how many neutrons does a OH- ion has?

O has 8 protons and 8 neutrons (16 molar mass - 8 protons)

H has 1 proton and 0 neutrons (1 molar mass - 1 protons)

so there are 8 neutrons and 9 protons in total

(this is assuming you are using the standard elements on the periodic table and not a weird isotope...)

and by "& how can it be lower than the number of protons of it?" you mean neutrons lower than the number of protons, it already is... but even less? add more hydrogen's, it will only add protons since it doesn't have neutrons, or if you want to keep the number of oxygens and hydrogens, just use a different oxygen isotope with less neutrons