let w = 20;

let h = 20;

​

function setup() {

  createCanvas(400, 400);

  frameRate(10);

  rectMode(CENTER);

​

}

​

function draw() {

  background(random(40,80));

​

  // do this 10 times

  for (let y = h; y < height ; y = y + height/num) {

    for (let x = w; x < width; x = x + width/num) {

      fill(255,0,0,random(255));

      rect(x, y, w, h);

    }

  }

}