Arsenic is in the group 15, so its atoms have 5 valence electrons.
Arsenic has atomic number 33. This means arsenic atoms have 33 protons and, if neutral, 33 electrons. The electron configuration for a ground state, neutral arsenic atom is:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3 or [Ar]3d104s24p3
By knowing the electronic configuration we can calculate the number of electrons present in the valence shell