Instead of trying to remember lots of different dot and cross diagrams, it may help to understand how to draw them using given information.
Atoms form covalent bonds by sharing electrons to get a full outer shell. This means that the number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell. For most elements, a full outer shell is eight electrons.
The table below shows the number of bonds formed by elements in groups 4 to 7.
Note: Hydrogen atoms only have one electron and form one covalent bond as they only need one more electron for a full outer shell. This is because the first electron shell is filled by only two electrons.
To work out how many circles to draw for a simple molecular substance and how to label them, look at the formula. For example, the formula for ammonia is NH3. For this, draw four circles, one labelled N and three labelled H. Each of the three H circles overlaps the N circle.
Nitrogen is in group 5 so it forms three covalent bonds. There are three shared spaces between the circles, so add a dot and cross to each one.
Finally add in the non-bonding outer electrons. Nitrogen atoms have five outer electrons. Three of these are shared, leaving two non-bonding electrons. Add these to complete the diagram (hydrogen atoms only have one electron, so they have no non-bonding electrons).
The number of non-bonding electrons is almost always a multiple of two, and it is usually much clearer (and easier to check when drawing a structure) to arrange the non-bonding electrons in pairs, to match the pairs of electrons in the covalent bonds.
Some molecules contain a double bond, which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms between them.
Nitrogen molecules consist of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons.