At an Edinburgh Fringe event, Scotland Secretary Ian Murray claimed that a wealth tax would lead to the rich fleeing the country.
Tax Justice UK have debunked this myth, showing that the vast majority of wealth won't be moved.
https://taxjustice.uk/blog/debunking-5-common-myths-about-wealth-taxes/
It was £20 billion when Labour took over, maybe Reeves needs a bigger abacus or one of those large size promo postage stamps to do the calculations on.