A crowd of that size would have been impossible for that time frame. Max Roser (whose site is fascinating) estimates the entire world population at around 72 million and notes that growth was very slow, almost nonexistent, for much of human history. If there really WAS such a crowd leaving Egypt, they would have left a significant footprint in the Moab desert. Say, for example, the mortality rate was just 1% (that is a relatively modern mortality rate and likely would have been higher in ancient times but for the sake of argument...). That means 24,000 people died every year, or 2,000 average per month for forty years (not counting the regular mass wipe-outs by the loving God). Evidence of funeral pyres or other burials and remains would be striking, offering undeniable proof of the WT's assertion. Yet, as others have noted, nothing.
Sorry, UA, you are right, we are off your point, which is a good one! Still, this was one of the facts that woke me up to the Bible's errancy/exaggeration and the Society's naivete.