It varies and they will never tell you ahead of time on their stock levels.
But if you really want one on launch day get up early... I am going to get to the Rogers store at 5am just so I can get one.... or maybe an Apple store still deciding
You decide on how busy your store is. If it is really busy then chances are a lot of people will go there if it is not so busy then you might get a good chance.
Not all stores get the same amount. Some might get 10, others 50. Only Rogers knows ahead of time. And those standing in line know the morning of when the store starts telling people to go away. But if you want one. Get up early