I currently make ~$130/month on Patreon, and have about 40 backers.
It varies from person to person! If you've got a massive following already, and you start your Patreon, you're likely to see more patrons early - but if you start your Patreon when you don't have thousands and thousands of followers, it'll be slower going.
I've had my Patreon running for about 2.5 years, and have gained most of my patrons in the last 7-8 months. I had a small handful of followers when I started it, though, and have been building my audience along with it. Unless you're mega-popular when you start your campaign, slow growth is the norm for Patreon!
I have no idea, tbh. Most Patreons other than my own that I pay attention to tend to be webcomic creators, and they create a wide span of different things. I don't think any of the ones I keep an eye on are BL creators, but they tend to do well anyway - but then again, if they're popular enough that I know about them, they probably have that established audience anyway.