how many new elements are left for anyone to discover?

A common trope in science fiction and UFO conspiracy theories is that aliens or lost civilizations discovered elements that we simply don’t have on the periodic table yet, allowing them to do whatever weird, unlikely, or even downright impossible thing they were supposedly doing. But how plausible is this? A lot of these elements need to be super-heavy, and that makes the whole concept very problematic given the dynamics of subatomic elements and fundamental forces…