There isn't really an upper bound, but the prime rate doesn't move much. As another comment said, competition between different lenders keeps the rates in check.
Typically the Bank of Canada sets their prime rate, some time later the big banks set their own prime rates based on that, then the mortgage rates are set based on that. The Bank of Canada prime rate only moves by .25% or .5% at a time.
If you have a variable rate mortgage and the rates change, they will be immediately reflected your mortgage. This isn't as bad as it sounds - your payment will stay the same, the rate change just affects how much goes to interest vs principal. The mortgage documents will include the 'trigger rate' which is how high interest rates need to get before your payments no longer cover the interest. This is the point where you're in trouble.
For some variable rate loans, like an auto loan, an increasing rate just means that the term of the loan gets longer or shorter.
As always, ask questions. The bank, in Canada at least, doesn't really want you to default on the loan. Ask about the trigger rate, ask what happens if it gets hit, ask what happens if rates go up but don't hit the trigger rate, ask about lump sum payments.
