Akechi Mitsuhide attacked Honnô-ji temple with around 12,000 men, although something I don't understand is how he convinced his men to do so.
He had been ordered by Oda Nobunaga to lead his troops down in the south. Instead he turned around and attacked the temple Nobunaga was residing at.
As far as I know, Mitsuhide and his men were all under the command of one man; Oda Nobunaga. Mitsuhide's men would have had to have known they were attacking Oda Nobunaga at Honnô-ji, and I really cannot think of a really good reason how he could convince 12,000 men to do something like this.
Plus, even afterwards what did Mitsuhide and his men think was going to happen after they killed Nobunaga? That Hideyoshi wouldn't just find and kill them all?
This is a question that's been bothering me for a while now, and I would like if I could get an answer for it.
I can't answer "How" he was able to do so.
However, it is quite clear that he didn't really need to. Akechi Mitsuhide was under the command of Oda Nobunaga. Mitsuhide's men however, wasn't.
When looking at Sengoku Japan (or many other places and times throughout history) you must dispense with the notion of a centralized military authority and loyalty. Armies were regionally raised and loyal to regional lords, not to a central lord (unless he also happens to be your regional lord). Unless Mitsuhide's vassal samurai for some reason felt more loyalty towards Nobunaga than Mitsuhide due to personal/familial ties or thought it was a good chance to rebel or something, the default is to just follow Mitsuhide.
As for what he thought would happen, we don't really know. Perhaps he wasn't thinking straight. Perhaps he thought that he, as one of Nobunaga's top generals, could rally enough support from old enemies and allies alike to make a successful bid for power. By all accounts Hideyoshi's speed marching west completely caught everyone off guard.
We have no way of knowing Mitsuhide's train of thought. But we need to remember this is the Sengoku. Successful or failing revolt against one's lord happened all the time.