Make array variable of all buildings(, if you need use “for each” node to set all of them to normal material, randomize number between 0 to 19 (or do whatever condition you want), read building from that index. here you have example but i used pawn class as example:
I did something extra i made it randomize between 0 and size of array, 2nd i don’t think you need setting material for all buildings, just make blueprint with already set material 
But i don’t know what you trying to make so ;p