Like if I used thief on his Emboar(in my case) would I get his focus sash forever? Also would he get anotherone next time I battle him and I can get as many as I want??
No, you can not.
Thief no longer steals a Trainer's Pokémon's items permanently; however, a wild Pokémon's item is still permanently stolen by the player. -Bulbapedia
so you could use the focus sash in the actual batte you used thief but you would not have it permanently.
