How can i tell how many neutrons there are in an element

It all depends on if you know the mass number and atomic number then you will be able to find out what the amount of neutron in an element.

Single replacement is a reaction in which one element reacts with another compund to form another compound and another element.

example:

A+BC > B+ AC

Double Replacement is a reaction in which the metals present in two compounds change places to form two new compounds 

example:

AB+ CD > AD+ CB (they basically switch partners)