I am confused on how you would write out this sort of formula

Sn3(PO4)2 ?

I don't get what the bracets() are for and why there is a number outside of it.

(NH4)2SO4?

Because there's 2 of the thing inside the brackets.

The brackets normally signify a group of elements that are found together. They are normally strongly bound and can be treated as a single species.