I saw via the BBC that there are over 100 candidates, including a dog, running for Toronto mayor. Despite looking at a couple of sites that claimed to cover everything one might need to know, I struggled to find the voting system involved. The site just linked mentions that a candidate could win with just 20% of the vote. In fact, it seems that the election uses first past the post - at least, according to Wikipedia, though no citation is given. This means that, in theory, one candidate could win with just 1% of the vote.
FPTP seems pretty undesirable when there are this many candidates. However, while some form of instant run-off could overcome some problems, it would also be burdensome on voters to rank all of the candidates.