It is a significant number. For example in Hardvard campus 20% of the allegations were determined to be false[0]; but this is a controversial subject and the numbers all over the place[1] from state to state and even more from country to country but overall most quote numbers that are not an insignificant number.

[0]http://www.washingtonexaminer.com/report-1-in-5-campus-rape-...

[1]https://en.wikipedia.org/wiki/False_accusation_of_rape