The first tier of upgrades is 3 workers. Oil production and SAM damage cost gold and can be reduced to 2 workers if you have Mansa Musa complete. Plane damage costs oil (10k for the first upgrade), so no worker reduction there. The upgrade times are relatively short but the cost is quite high. I just started tier 4 of oil production, it was 2.8 million gold and takes something like 16 hours. Yes, hours, not days. If all the researches are like this, Amelia is going to be complete quite fast. More like Mansa and Hiawatha than like the others.