Arsenic, #As#, is located in period 4, group 15 of the periodic table and has an atomic number equal to 33.
This means that a neutral arsenic atom has 33 electrons surrounding its nucleus. The electron configuration for arsenic looks like this
#"As": 1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6) 3d^(10) 4s^(2) 4p^(3)#
or, using the noble gas shorthand notation
#"As": ["Ar"] 3d^(10) 4s^(2) 4p^(3)#
As you can see, the outermost shell is the fourth shell, #n=4#. This shell contains a total of 5 electrons, 2 in the 4s-orbital and 3 in the 4p-orbitals.
Since arsenic is not a transition metal, its valence electrons will be those electrons located in its outermost shell #-># 5 valence electrons.