Hashtags are essentially search terms so you want to find a sweetspot where you use something people might search like "inktober" "sketchbook" "[yourcity]artist" .etc, but not something so vague and overused that it will be drown out in the search results like "art" and "drawing". I'm by no means an expert but this is my understanding of it. There's also nothing wrong with using a couple really random obscure tags and seeing who else has that same random idea - it just won't lead to great visibility. I tend to hashtag my stuff a combination of things relating to the medium, subject matter, myself as an artist/my city/state, and then special interest groups that I've searched out beforehand. For instance if you're working in ink "inkedartgroup" and "inklouvre" are both moderatly popular tags to attract fellow artists. If you're illustrating a pitbull you could go with pitbull-centric tags like "pitbulllove" and "pittienation" to attract pitbull lovers that might enjoy your work. Always think of it as knowing your audience/who you want your audience to be, and what they might search for.
