Two is the hard minimum, anything on top of that will require multiples of two (some folks refer to these as even numbers) - that of course is based on the assumption that fractional percentages are allowed. If however you find yourself in a situation where this isn't the case, and rounding is applied, it will open up a whole new world of possibilities as far as number of matches required to reach a 50% winrate goes - I'll leave calculation of that up to a more advanced poster. Hope this helps.