Upvote:12
The stamp you get saying how much time you have is valid only for that visit. If you leave the US, that stamp no longer applies. Since you have a multiple entry visa, you will get a new stamp every time you enter the US.
If you are genuinely living in Canada, and are visiting the US using your B2 for short visits, then that will likely be perfectly acceptable to the US immigration officer. They'll probably just stamp you in for another six months every time you enter, even if you have only planned a short visit.