let fr = 3;
let circle_xs = [];
let circle_ys = [];
let num_of_circles = 500;

function setup() {

  createCanvas(400, 400);
  background(220);
  frameRate(fr);

  // render random circle position at start
  for (var i = 0; i < num_of_circles; i++) {
    var circle_x = random(400);
    var circle_y = random(400);
    circle_xs.push(circle_x);
    circle_ys.push(circle_y);
  }
}

function draw() {
  background(220);

  // draw circle according to the list
  for (var i = 0; i < num_of_circles; i++) {
    circle_x = circle_xs[i];
    circle_y = circle_ys[i];
    circle(circle_x, circle_y, 5);
  }
}