Darski, increasing the appearance number should increase the likelyhood of appearance, however, I set the disappearance number to zero so the iron does not disappear. I need to check that to make sure about the increase. However, the change will only apply to the .biq file that you generate, and will not apply to maps generated to start a game. I have run into the same problem with only one (1) source of a strategic resource on the map. The problem when that happens is that you really only find out that is the case once you can see the entire map. By then, you have a fair amount of time invested in the game. That is one reason why I like having an editor and use it to generate maps, and then look them over.
Besides changing the likelyhood of appearance, you can also as a backup, position some iron on the map yourself. I would recommend on edited maps setting the iron disappearance to zero, then once you have it you cannot loose it.
My general rule when using the map generator at the start of the game to produce a map is if I cannot get iron within the time it takes me to get to the medieval period if I am researching at a fast rate, or by the time I have about half of the Ancient Tech Advances researched if I am slow, I quit the game without saving and go on to another one. Since I have the editor for Conquests on my Windows editing box, I have been using edited maps or scenarios pretty much exclusively. As I have said before, I am not worried about making the Hall of Fame or anything like that. I just play for my enjoyment.