Are there any guidelines for labeling axes in plots/graphs when the "unit" is "number of people"? I have read the question Are there any guidelines for labeling axes in plots/graphs? and related answers and they cover the case where the unit is from the International System of Units (SI), but what about when the graph shows information about people?

I am interested in medicine or in psychology where a lot of graph are related to number of people, is there any wide adopted guideline?

What is the right word to use?
People?
Person?
Individual?

For example if person were a unit of measurement I should write 10 person and not 10 persons because for SI units we do not use plural forms.

In the same way, in a axis label I should write person and not number of persons because we usually write meter (if not abbreviated) and not number of meters.

And then, what is the short form for person? I see sometimes pax but as a short form for passenger of an aircraft.
You always have to distinguish between the quantity and the unit. 

All the quantities referring to a number of entities have unit 1, so you don't need to put any unit after the number.

About how to call such kind of quantities, you're actually relatively free to decide, depending on the specifics of the case at hand. You can certainly call a quantity that counts certain elementary entities X as number of X. But, in case, don't use amount of X because that has a specific meaning in chemistry.

Examples of quantities called number of X can be found e.g. in the IUPAC Green book: 


  N is the number of identical (periodic) events during the time t. (p. 13)
  bi gives the number of electrons which occupy a given orbital energy level εi (p. 19)


Even better, if that number is a recurring quantity in your paper, label it with a symbol and use that symbol to label the axes: "Let nr be the number of people who always wear red trousers [...]".