How to add random number of falling balls in pygame?

Here is some code for random falling balls, I am able to generate one falling ball. Having hard time to randomize the number of falling balls, also they need to be some random delays falling from "sky". Could you tell how to do it ?

import pygame
import random

pygame.init()
width, height = 640, 480
screen = pygame.display.set_mode((width, height))
 
robot = pygame.image.load("robot.png")
 
y1 = 0
y2 = 0
speed1 = 1
speed2 = 2
 
clock = pygame.time.Clock()

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            exit()

    screen.fill((0, 0, 0))
    robots = []
    screen.blit(robot, (100, y1))

    y1 += speed1

    if y1 >= 640:
        y1 = 0

    pygame.display.flip()

    clock.tick(60)