First of all, we need to clear up what a covalent bond is.
A covalent bond is a bond of the family of the chemical bonds. This means that atoms bind together. I know from my chem classes that, in the end, an ionic bond is just an extreme case of the covalent bond, while a pure covalent bond occurs between the same type of atoms. So that takes us again to what a covalent bond is.
Imagine you just have ONE hand. The you meet another person with just ONE hand. Well, if you are feelingly lonely, why not hold hands? Atoms do similar things. Pretty much every atom, except the noble gases (with some exceptions) establish bonds. The three main types are:
The metallic bond is a wierdo. It usually appears between two similar atoms. We can say that almost every metal exists bonded by metallic bonds.
The ionic bond, as mentioned before, is just an extreme case of the covalent bond, where the electronegativity of one atom is 2.0 points or more above the electronegativity of the other atom in the bond.
The covalent bond is simply a bond where two atoms share electrons. They say: hey, take this electron and you give me one. That is a simple covalent bond. If they say: hey, take two electrons and you give me two, that's a double covalent bond. And so on. I believe that the max number of shared electrons is 4, but those bonds are waay too unstable.
With that said, let's analyze the propane.
Count the number of lines between atoms. Those lines represent a pair of shared electrons. Basically, you just count the number of lines in the compound and you get the number of covalent bonds.