@TumeeTum its somewhere in the last chapter of Vol. 2 which is the latest translated volume if I remember correctly so you're only gonna get a few more sections with that. If you still want to know what's next, there is the original Web Novel version which has a couple arcs you can read.
@fairestflower nothing could ever beat the WN version, there are too many details that can't be expressed through video, and is better to be left imagined.
Just in case if anyone is curious, it doesn't say yet how many girls are in Shadow Garden nor will the manga say it soon, but as of Rose joining it there are girls in Shadow Garden.