Question

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??

Answer 1

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.