Durning #beta^-# it increases by one. Durning #beta^+# it decreases by one.
Durning #beta^-# a neutron turns into a proton, an electron and anti-electron neutrino are emitted in the process. The electron is the beta particle in this reaction. The nucleus gains a proton in this reaction so the atomic number increases by one.
Durning #beta^+# a proton turns into a neutron, a positron and electron neutrino are emitted in the process. The positron is the beta particle in this reaction. The nucleus loses a proton in this reaction so the atomic number decreases by one.