You just have to create a variable and increase it each time the mouse is pressed:

Code:

int counter;

void setup() {
  size(100,100);
}

void draw() {

  
}

void mousePressed() {
  counter++;
  println(counter);
} 

'counter++;' is a shortcut for:

counter = counter + 1;

Note that there are different ways of handling mouse press (see the Reference) and which one you choose depends on what you want to achieve.  The function outside of draw (as used above) would be used generally if you want something to happen only once each time the button is pressed.  The mousePressed variable might be used in draw to make something happen for as long as the button is pressed.