What is the atomic number of Mn?
Mass number is essentially the number of protons in the element. Remember that protons are what define an element. 25 protons is the atomic number of Manganese, and 12 is the atomic number of Magnesium.
The element to the left will always have an atomic number one less, and the one to the right will always be one more.
In fact, the periodic table is in numerical order by Atomic Number. They'll always be whole numbers, 1-118.