On 4chan post numbers ar distributed throughout all threads. So in a single thread, you'll see numbers skipped. 101, 126, 178, 221. Which means 777 could be plausibly skipped. For example, in that photo, you see 777 skips to 831. There were 54 posts elsewhere between the two replies. And the 350 reply limit still stands, so you will only get 350 numbers between 001 and 999. But the commenter wasn't entirely right, you are pretty likely to get some repeats. So a little less than 30%.