Click the green flag, and when it reaches a million - you'll hear a sound. Want to go faster? Try it in "Turbo mode": press the Shift key while clicking the green flag.
Credit to @MrShah for the "Random Dots" project created in 2007, the first year of Scratch. I had originally remixed as a million stars, then a million cats, and now a million cats and thanks! I added a variable to keep count, so you can see how many cats have been painted so far. It repeats until the counter equals 1,000,000 (one million).