Different sources claim different numbers, but the rate is considered by most sources to be low.

While the statistics on false allegations vary – and refer most often to rape and sexual assault – they are invariably and consistently low. Research for the Home Office suggests that only 4% of cases of sexual violence reported to the UK police are found or suspected to be false. Studies carried out in Europe and in the US indicate rates of between 2% and 6%.

https://theconversation.com/heres-the-truth-about-false-accusations-of-sexual-violence-88049

Their arse.