You don't need active moderation up to around 10000 users, but you should have code that handles blocking by Day 1.
There's a discord game out there where someone tries to put in random strings in a link sharing site and see who gets to interesting content the fastest. I'd say the NSFW content is is 1 in a hundred, but this is anectodal data. Actual illegal content would probably have been taken down earlier, but enough of it slips through things like MEGA that the FBI probably won't hunt you down for it.