This article from July 2020 reports the NEC confirming the number as 570,000.

More people have joined the party than left under Sir Keir Starmer’s leadership, Labour’s National Executive Committee has confirmed.

https://www.thelondoneconomic.com/politics/many-more-people-have-joined-the-party-than-left-labours-nec-confirms/28/07/

If it's now around 495,000, that's 75,000 gone in 4 months.