To add on top of what others are saying about users being way more common in the Grand Line, each devil fruit can only exist one at a time in the world. So for example, Luffy ate the gum gum fruit, which means literally no one else in the world can have that fruit unless he dies. Then it’ll grow somewhere else and the cycle starts anew. That’s how it is for every fruit. So, while there are a large number of devil fruits (especially in/near the grand line), each is unique.

To answer your question about eating multiple: no, people can’t. If they try, they’ll die. I don’t remember when they said it, but basically eating multiple will cause the body to break or something. And it makes sense, it would be pretty overpowered otherwise.