Japan could send a large army to Europe

How large an army could Japan have sent to Europe during the First World War?

The estimate is definitely hundreds of thousands and probably the low millions, based on Japan's population and army size.

Japan's existing army and population are the basis of this estimate. The answer begins with millions, then qualifies that to definitely hundreds of thousands and probably the low millions. It gives an approximate scale rather than a detailed calculation of an expeditionary force.