Most founders post in r/startups and r/Entrepreneur and wonder why nothing happens. Those subs have millions of subscribers but the people in them aren't your customers. They're other founders looking for the same thing you are.
The subs that send signups are smaller, weirder, and harder to find. Here's how to look.
Start from the problem, not the audience
Don't ask "where do my customers hang out?" That gives you the obvious subs and they're saturated. Ask "what problem does my product solve, and where do people complain about that problem in detail?"
If you're building a tool for indie hackers who run paid ads, you don't post in r/Entrepreneur. You post in r/PPC and r/marketing where the actual frustration lives. The people there are smaller in number but every one of them has felt the pain.
Look for active mid-size subs
The sweet spot is roughly 20k to 200k subscribers, with at least a few posts a day. Bigger than that and your post drowns. Smaller and there's no one to see it.
You can eyeball "active" by sorting by New and looking at the timestamps. If the top of New is from yesterday, the sub is dead. If it's from twenty minutes ago, it's alive.
Read the rules before you read anything else
Some subs ban self-promo entirely. Some allow it on Saturdays. Some require flair. Some auto-remove anything with a link. The rules tab tells you in two minutes whether a sub is workable.
If a sub bans self-promo, don't write it off. You can still comment there and link to your site in your profile. Reddit profiles are a real channel that almost nobody optimizes.
Build a short list, not a long one
Three to five subs is enough. More than that and you'll spread thin and stop showing up anywhere consistently. Pick the ones where your problem is most painfully discussed and go deep there.
What deep looks like
Deep means the mods know your handle. It means you've answered the same five questions enough times that you can paste a polished version. It means when you post, three or four people upvote within the first ten minutes because they recognize you.
That's worth more than any front-page post in a giant sub.