Could you explain more about what you are trying to accomplish? Using a String to determine a path for an object is not a default function in Blueprints. You could code a custom node to do that though. There are many ways to swap 50 or more meshes, but that all depends on how you would like it to function.
Are the meshes something procedural and random, or would a level designer just need an easy way to swap between them? Can you give an example of something similar?