It is hard to know for sure the exact number but from what I've seen, false rape claims are generally thought to be under 10%. There are plenty of actual rapists that get off because the victim is afraid to testify or because there isn't enough evidence or because of rape culture stuff about girls getting drunk or going to nightclubs.
Its such a difficult one to talk about, but I think one thing that we can all agree on is that we want more rapists in jail, and for women who have been the victim of rape to come forward and testify.
Slightly unrelated note, I think that the current feminist line of 'rape is rape' is actually quite unhelpful to the debate, and isn't helping their cause. If we examine four different scenarios:
1. Woman next day feels she was too drunk to give consent legally, but has come to believe that she did in fact give consent.
2. Woman next day feels she was too drunk to give consent legally, and can't remember if she did.
3. Woman next day feels she was too drunk to give consent legally, and is certain that she did not.
4. Woman is raped by stranger on way home, with no alcohol involved.
All of these would be considered rape, but I think that it is natural for us to feel different levels of sympathy for the woman involved and different levels of anger towards the perpetrator.
Basically I think that its an issue that's been hijacked by a quite forthright feminist group in society, and thus having real discussion like Ken Clarke tried a couple of years back is seen as completely unacceptable by the media. If however we want to see more of the people who are genuine dangers to the women of our society behind bars I think its a conversation that needs to be had.