How many people show up and say "nope" on the record instead of staying home.
I have no idea. I suspect some people don't even know that it's an option they have. That's one of the things I've been mentioning whenever voting procedures come up in the conversation on CBC.ca. They also tend to be unaware that in municipal elections, they're told to vote a full slate of councilors and school board members, but they're not legally
required to. It's been many years since I've bothered voting full slates, after realizing that doing that could well mean canceling the vote for the one or two people I really want to get in.
It probably happens more in municipal elections than provincial or federal. In municipal elections, there are separate ballots for mayor, councilors, school board (voter states which they're voting for - public or Catholic), and there might be an occasional extra vote for some question or other (ie. plebiscite piggybacked on to the election, to save time and money instead of having a completely separate vote later). We used to have hospital board elections as well, though not for a long time now. Citizen hospital boards were disbanded years ago.
That said, there were plenty who declined their "Senator-in-waiting" ballots when those were piggybacked on to one of the municipal elections. They just couldn't see the point in voting for someone to be appointed to the Senate when the vote wasn't binding, and the only people running were either Conservatives or independents who everyone knew was either Conservative or from a right-wing fringe party. I'm more aware of that situation since I was a DRO for that election and responsible for handing out the ballots and counting them after the polls closed. I myself declined the Senate ballot because it was both pointless and there's no way I'd vote for anyone on it anyway. To boot, one of the people on the ballots had just died... but apparently some people didn't bother reading the newspaper, since they voted for him anyway.