Warmech is battle 0x56(a), only shows up on Sky Castle 5F, and there's only one of him. No battle in the game has more than one Warmech at a time.

Random encounters in the NES version aren't very random; they're read from a fixed table. You're guaranteed to find Warmech after 129 or fewer battles in a row as long as you don't turn the NES off. Usually fewer, depends mostly where in the encounter table you are when you start counting.

The NES version of this game has been thoroughly dissected. See any FAQ about FF1 battle mechanics, e.g. http://www.gamefaqs.com/console/nes/file/522595/57009. Or find a ROM and check using FFHackster (http://disch.zophar.net/ffhackster.php). The screenshot is fake or from a hacked ROM.