How do you determine the correct subscripts in a chemical formula? What is a subscript?

Chemical formulas are a way of expressing the chemical ingredients of a compound or molecule. Molecules are made up of one or more of the same elements. The abbreviations for these elements come from the periodic table, so you should keep the periodic table handy to help you identify them. The subscripts in a chemical formula are the small numbers that you may see written in the formula. These subscripts indicate the number of atoms of each given element that are contained in the compound. 
Consider a very common chemical compound, water. The chemical formula for water is H_2 O.
In the chemical formula for water, the subscript for hydrogen is 2. Notice that the 2 is smaller and written slightly below the H and O. It is called a subscript because it is written ("script") "below" ("sub") the preceding letter.
The subscript 2 simply indicates that the chemical compound water has two atoms of hydrogen (elemental symbol "H"). The compound water also contains one atom of oxygen, which is indicated by an O with no subscript. If an element has no subscripts at all, you can assume that there is only one of each atom in the molecule. Consider the compound CO, also known as carbon monoxide. Carbon monoxide is made up of one atom of carbon (C) and one oxygen (O). 
A more complicated chemical formula can be found for the compound ammonium sulfate. It looks like this:
(NH_4)_2 SO_4
The parentheses group the atoms together. The subscript 2 after the parentheses means that ammonium sulfate consists of two ammonium molecules (NH_4), each of which is made up of one nitrogen atom (N) and four hydrogen atoms (H). Sulfate, the other component of this molecule, is made up of one sulfur atom (S) and four oxygen atoms (O). 
See the references for some common chemical formulas.
P.S.: You may also see superscripts in a chemical formula. These are like subscripts except that they are written toward the top of each element rather than the bottom. Superscripts indicate the charge of an ion and will have a + or - in front of them to indicate whether the charge is positive or negative, as well as a number to indicate the charge's magnitude. For example, consider (NO_3)^- . This is the symbol for nitrate, an ion (charged molecule) consisting of one nitrogen and three oxygens. The minus ("-") at the top of the formula indicates that this compound has a charge of -1. Superscripts can also use numbers to indicate higher or lower charges. 
https://periodic.lanl.gov/index.shtml

https://www.thefreedictionary.com/chemical+formula