If this is corn as in vegetable and not a houseplant called corn plant which is actually a tropical foliage plant, you need to understand that the seedlings might grow to a certain point but is not likely to reach potential as crop bearing plants.
12 inches might sound tall/good, but I wonder if if it is growing tall from lack of light? Outside in the garden, corn is a field crop that need all day direct sun if at all possible, which is not possible to replicate in an average home.
If you are growing these just for fun and experiment, by all means. Tell us what size containers they are growing in right now, and your future plans. What variety corn they are so we know what the ultimate size would be.
As a basic watering tip: (1) You can feel the surface of the soil, and if it feels dry when you poke your finger in to first knuckle, it needs to be watered (2) you can do this with a wooden or bamboo chopstick -- if chopstick comes out dry, then it needs to be watered (3) or you can lift and feel the weight (heft) the container and it will feel light compared to immediately after watering the last time.
When you water, water until the pot drains out of the holes in the bottom, then if this water is not absorbed back by the container within 30 minutes, drain the drip pan so the container doesn't sit in water.
Learning never ends because we can share what we've learned. And in sharing our collective experiences, we gain deeper understanding of what we learned.