Sauron's great mistake

[Gorthol]

I think the answer lies maybe within *what* the numenoreans were longing for, everlasting life. They thought they could wrest this with force from the Valar. Remember that Ar-Pharazon was already "Lord of Middle-earth" as far as anything goes.. Fighting elves in Middle-earth would not bring him closer to his goal of becoming imortal.

At length Ar-Pharazôn listened to this counsel, for he felt the waining of his days and was besotted by the fear of Death. He prepared then the greatest armament that the world had seen."

Sauron also despised the race of men and I don't think he wanted more interaction with them than what was absolutely necessary.

Elendil did leave Numenorë with only 9 ships, hardly more noticable by Sauron than "the Witch Kings" reaction of Master Samwise's crawl in the mud "like a worm".