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.
It also depends on your site. For example if you display content in chronological order like Twitter, then you incentivize people to spam as their content is immediately visible. I.e. my first post will likely be "test" - if I see it in the homepage then I delete it and post a link to any of my projects (if not worse).
Finally, moderation shouldn't be massively complex at the beginning. Just implement a super downvote (that maybe only you/your friends can see) that kills content you don't want to see with 1 click.
Good luck, looking forward to a show hn!
My idea is basically a Reddit-but…
So it would work great with the same exact content as Reddit itself.