The 120,000 troop deployment is the third option. The number is too high for an attack by air and on sea and too low for an attack on land, i.e. an invasion of Iran. Releasing the third option number is likely designed to rally against such a move.
From there on varying numbers were all over the news:
Unfortunately, it was still wrong. Here is the real one:
Acting Defense Secretary Pat Shanahan notified Congress on Friday that he had authorized a request from U.S. Central Command to send the additional forces – an Air Force fighter jet squadron, an engineering element and combination of manned and unmanned intelligence, surveillance and reconnaissance assets – into the Middle East, he said in a statement. The Pentagon will also extend the deployment of some 600 soldiers from a Patriot missile battalion already serving in the region.
There seems to have been a minor war between John Bolton, Trumps National Security Advisor, and the State Department or Pentagon. Bolton is longing for his war on Iran and sending lots of troops would eventually provide for that. The State Department and the Pentagon want to prevent that catastrophe from happening and preferred to send none. Trump eventually agreed to a minimal number.
There are in total some 20,000+ U.S. troops in the various countries of the Middle East. The additional 900 send now will not make any difference.
The idiocy of the whole discussion was well captured by the German satire site Der Postillon. On May 16 it headlined (translated):
Tensions between Washington and Tehran continue to increase. Now the United States accuses Iran of moving its borders ever closer to US troops. In part, American soldiers were almost in firing range.
“Iran is using its territory to harass our peacefully deployed troops,” said US National Security Advisor John Bolton. “Some of the country’s borders come dangerously close to our soldiers – almost in range, Iran must stop doing that right away!”
The US threatened to mobilize its troops. Bolton: “We are a peaceful nation, but if Iran dares to let its borders cross below the feet of our soldiers, that inevitably means war!”
Five days after later Reuters converted the satire into news: