More common than you think, particularly at lower levels.
There's a lot of collisions in low-level beer league where people have their heads buried, don't see each other coming or maybe aren't completely controlled stoppers. Those all cause head vs. ice collisions, something your head almost always loses.
I've had one diagnosed and two probable concussions, all very minor, in six years of rec hockey. First probable was an open hockey session where two guys collided and fell and slid down the ice, one taking out the back of my legs and whipping me back to the ice where my head smashed. First diagnosed was a collision with a much larger human where we tried to avoid each other but both moved the same direction and read head on into each other which sent both of us onto our backs. That also caused a strained neck. Second probable was a year later where I was trying to collect a puck and a guy came to pressure me, but he couldn't stop and bowled me backwards really hard. He got called for that, but it was unintentional.
Some of it depends on how tight the refs on your league call incidental contact (the refs in my league = you have to be really reckless or accidentally blow someone up to get called). The tighter they call it, the fewer collisions and, therefore, the fewer concussions.