If a tournament has X number of players. What are the odds of winning the tournament?
Assuming random hand distribution and players of equal ability, the probability of winning is 1/x. Of course other factors are involved in real poker, so it's impossible to summarize this in a simple formula.
If you are interested in the more specific question of how much cash equity you have in a tournament given the remaining players' stack sizes, you might want to look into the Independent Chip Model (ICM). It's useful as a way of determining the cash value of your current stack, and the value of your decisions, particularly later in a tournament when the money ladder is in play.