So, some quick calculations. A Liberty ship troop transport can carry about 500 troops at about 28 tons per soldier. Assuming 10,000 men per division, 20 divisions would require 5.6 million tons of shipping.
So since Japan had 6.5 million tons of shipping in 1941,
67th Tigers is right that it could ship 20 divisions to the West Coast.
If Japan had that much shipping in the 20s. Assuming those troops don't need heavy weapons or vehicles. Or food/fuel/ammunition. Or landing boats, since those troops will be disembarking into a friendly port. Or air support/engineers/any other assorted personel an army needs besides actual combat troops. And those ships don't need more shipping to carry enough fuel for this long, long voyage. And the Japanese economy and the Navy can still operate while splitting the remaining 900,000 tons. And if assuming all that remaining shipping goes to the Navy, the Navy would be capable of conducting operations near the West Coast when it could barely do the same near Hawaii OTL with twice as much. Which of course wouldn't be a problem, since that big convoy of troop transports doesn't have anything to fear from the USN.
So since Japan had 6.5 million tons of shipping in 1941,
If Japan had that much shipping in the 20s. Assuming those troops don't need heavy weapons or vehicles. Or food/fuel/ammunition. Or landing boats, since those troops will be disembarking into a friendly port. Or air support/engineers/any other assorted personel an army needs besides actual combat troops. And those ships don't need more shipping to carry enough fuel for this long, long voyage. And the Japanese economy and the Navy can still operate while splitting the remaining 900,000 tons. And if assuming all that remaining shipping goes to the Navy, the Navy would be capable of conducting operations near the West Coast when it could barely do the same near Hawaii OTL with twice as much. Which of course wouldn't be a problem, since that big convoy of troop transports doesn't have anything to fear from the USN.