Couple of problems with this:
- in draw() you're indexing into your array with carNR so you'll only ever draw the last car
- using global variables like carNR inside your classes will work but it's not a good idea if you want to avoid problems down the road
- if you want each car to know how many cars there are in total then give them a local copy of that information and update that (I've not done this in the code below)

Main sketch:

1. import controlP5.*;
2. ControlP5 controlP5;
3. PFont font;
4. int X = 100;
5. int Y = 65;
6. Car[] car;
7. int carNR=-1;  //initialise to -1 at the start to prevent
8.               //access before we've created something
9. 
   
10. void setup() {
11.   size(400, 400);
12.   smooth();
13.   frameRate(30);
14.   controlP5 = new ControlP5(this);
15.   controlP5.addButton("AddPlayer", 0, 20, 50, 60, 20);
16.   font = createFont("Helvetica-24", 12, true);
17.   textFont(font, 12);
18.   background(0);
19.   car = new Car[15];
20. }
21. 
    
22. void draw()
23. {
24.   background(0);
25.   for (int i = 0; i<=carNR; i++ ) {
26.     car[i].player();
27.   }
28. }
29. 
    
30. public void AddPlayer() {
31.   if (carNR<14) {
32.     carNR++;
33.     car[carNR] = new Car(X, 65 + (20 * carNR), width, height, carNR);
34.   }
35. }

Class:

1. class Car {
2.   int x, y, w, h, pnum;
3. 
   
4.   Car(int x, int y, int w, int h, int pnum) {
5.     this.x = x;
6.     this.y = y;
7.     this.w = w;
8.     this.h = h;
9.     this.pnum = pnum;
10.   }
11. 
    
12.   void player()
13.   {
14.     fill(255);
15.     textAlign(LEFT);
16.     textFont(font, 15);
17.     println(y);
18.     text("Player:"+pnum, x, y);
19.   }
20. }