int click  = 0;

int [] xy = new int[25];
void setup()
{

for (int i = 0; i < 20;i++)
  xy[i] = -1;
size(500,500);  
}

void draw() {
background(122,122,122);
  
for (int i = 0; i < 10;i++)
if (xy[i*2]!=-1) rect(xy[i*2],xy[i*2+1],50,50);
}

void mouseClicked() {

xy[click*2] = mouseX;
xy[click*2+1] = mouseY;
click = click + 1;

if (click == 10) click = 0;
}