How do I find the mass number when I dont know the number of neutrons?

You will have to look As up on a periodic table (or any element for that matter).

Like you stated, As has an atomic number of 33.  This means that it has 33 protons.  Each element has a unique number of protons and that is how atoms are distinguished.

On the periodic table, As has a mass number of 74.9 (the numbers are not whole numbers because of isotopes).  You will be safe a majority of the time if you just round. So we'll assume the mass number is 75.

If the mass is 75 and there are 33 protons then you have (75-33) = 42 neutrons.

Side note:  You may encounter questions where it says you have an isotope of As and it would have to specify a different mass number.  For example, As has an isotope with a mass number of 76.  In this case there would be 43 neutrons. There are of course still 33 protons, because it wouldn't be As anymore without 33 protons.