How you "kill" the iron golems so more can spawn.
You have to "kill" an iron golem for more to spawn. For it to count as dead it must leave a 30 block area around the villagers. You can do this by accually killing the iron golem or dropping it over 30 blocks down or even just pushing it into a nether portal. Nether portals are by far the fastest method to accomplish this because iron golems can spawn in them and instantly teleport. Most simple farms use lava to kill even though it decrease farm rates. Unless you have obsidian don't bother trying to make a farm more efficient. You get obsidian so early in the game that it isn't worth it to dig a hole to just place a few portals later.