Those would appear to be bots. You also see it from time to time from fake music channels.
To avoid getting these seen publically, I recommend you add into your bad words filter 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, this will prevent any comment with a number from being shown publically, and all those posts will have to be manually approved first.
Also, it could be possible that some channels that have been compromised (FPT was a recent one who luckily got his channel recovered) could be sending out bot comments in the hope you go and check them out for spam reasons.
Either way, I'd delete and hide any comments with numbers that don't look human.