Banff is genuinely good bear country: Parks Canada estimates about 65 grizzly bears, and recent research suggests a stable-to-slightly-increasing trend toward 70. There are even more black bears than grizzlies — despite a myth to the contrary, the best Bow Valley study found black bears outnumber grizzlies by roughly three to two. But bears roam huge territories, so a sighting is always a matter of odds, not certainty. Many visitors see none; others round a bend onto a grizzly grazing in a berry patch.
What moves the odds in your favour: go in spring or early summer, when bears feed low in the valley after hibernation (or in September, when they fatten up on berries), look at dawn or dusk, and let a guide who tracks daily sightings choose the route. Bears hibernate from about November to April, so winter sightings are very unlikely. If a tour promises guaranteed bears, be sceptical — no honest operator can.