You're in the right category, don't worry. Promotions is for promoting comics, and a lot of people on the forum have muted the whole category because people spam-post in tons of threads at a time, drowning out questions like yours or interesting discussions. 
That said, posting on some promo threads (though I'd personally advise avoiding sub-for-sub, because organic growth is better and more sustainable, and it's easier to gauge performance if you don't boost it artificially) can be a good idea to get you started.
For what to expect with sub growth, it's best not to go in with expectations. If it's your first time publishing a webcomic, assume that if you manage to get some readers who leave comments, you're doing great. Try not to get anxious over that progress bar on the dashboard and try to rush up to 25/100/250 subs as fast as you can; just focus on making a good comic, do some promotion on the Tapas forums and discord and social media, and treat it as a data-gathering or learning experience.
Your comic looks good. It's nicely presented and looks like it will read well on mobile, so I think if you just keep making your weekly updates and do a bit of promotion, you'll get readers. I'm not going to tell you to expect any sorts of numbers though, or tell you "if you don't have this number after X months, you're doing badly", because it varies so much by things like genre, style, story content, humour, how well a person promotes etc.
