It isn't a lie Karen. I was confused about this too until I listened to More or Less on the BBC earlier this week, which went into it in some detail.
According to the Kings fund, apparently we have around 280,000 nurses at the moment and the Tories are saying they will increase it to 330,000. By anyone's maths, that is 50,000 more nurses and "More or Less" confirmed there was nothing dodgy in the claim at all, let alone it being a lie.
"We don't think there's anything dodgy about this promise. It means what it seems to mean".
https://www.bbc.co.uk/sounds/play/m000bx15
Over the past 5 years, we have recruited 160,000 nurses into the NHS, but unfortunately we have also lost about 160,000. Over the next 5 years, the Tories wish to increase the first number and reduce the second one. For a net gain of 50,000 more nurses.
Now, you might very reasonably question whether this is achievable, and who knows, you may be right. But the numbers do add up. The Tories are promising 50,000 more nurses: 280,000 rising to 330,000.
I think they explained this spectacularly badly however.