Setting Texture on Terrain at x,y coord in runtime

How can i set texture on terrain’s x,y coords?
If there is no option to do this, then my question is how can i put multiple textures on 1 drawcall mesh , i mean if i created a quadratic mesh is there any possibility to put different textures in various squares(without doing it on 2 or more drawcalls)? I realised that there is an option to cut this mesh in places where textures change to dodge high amount of drawcalls(while doing it without mesh, only with quads)