If you're looking for hard numbers on how many people shouldn't be getting them then you won't find it. Only the government has access to the details of individual claims.
However you can infer a lot from a) the insane rise in claims, especially mental health related:
https://obr.uk/docs/box-chart-3-f.png
Has the mental health of the nation got twice as bad in 2 years? Obviously not.
And b) whenever the BBC does touchy feely profiles of people there are always some weird red flags:
https://www.bbc.co.uk/news/articles/cx2gpl4528go
Ā£400/month help with her bills because she struggles with time management? I'm sympathetic to her problems but that is a shit ton of money!
Even some of the people receiving it agree:
> "I was shocked by the ease with which it was granted. I was expecting to be interviewed, rightly so, but it was awarded without interview and he received backdated pay for the maximum amount."
>
> She was also surprised that her husband got mobility allowance for not having a car, even though she had a car and could drive him around.
(This reminds me of WFA where plenty of people receiving that also thought it was ridiculous.)
https://www.bbc.co.uk/news/articles/cn0ry09d50wo
> Paul Harris, from Barnard Castle, gets £72.65 a week in PIP payments to help with extra costs associated with his anxiety and depression - such as for specialist therapy apps and counselling.
https://www.bbc.co.uk/news/articles/cn4llx4kvv8o
> Nick Howard, 51, from Cambridge, is neurodivergent and has been claiming Pip for five years.
>
> "Without Pip I would not be able to work as it pays for my transport to and from my workplace.
>
> "I'm currently buying an electric bike on credit, others I have had have been stolen or vandalised," he added.
Great... but I don't think paying PIP for 5 years is a good way to buy someone a bike.
Obviously not all cases are like this, but clearly something has gone wrong. And this isn't a partisan issue. Both parties agree that it has to change. The Tories just ignored the problem and Labour gave up after predictable "N people will die!" press.
And to be clear I'm not anti-poor or anything like that. I also thing WFA is ridiculous and that mostly goes to the rich. Child benefit also goes to lots of people (myself included) who totally don't need it. They all need reform, but look what happens when the government tries...