I believe it was well over a million. Over 600k men (of fighting age). So counting the women and children -- probably close to two million. A bunch.
I wonder. The population growth f the world in 1 BC extrapolating from Roman, Gupta, Parthian and Han sources was 100 million. 500BC at the time of Cyrus the great was 50 -70 million. I would say it was 50 million at the time of the exodus (around 1500-1200 BC). 400K seems more plausible considering these as the remnants of the Hyksos