This question already has an answer here:
I have subdivided a plane into 50 x 50 faces, then I random selected some of those faces and saved them as a group, I hid them with h and random selected among the other faces and made them another group.
I repeated this a couple of time, so that I had one objected which contained several groups of faces.
From this point on, I tried to select one group and assign to them one material and to another group another material and so on...basically to achieve a floor of tiles with different kind of tiles.
But, it did not work.