It only takes one healthy seed to grow a catalpa tree. To grow them from seed, get one of the seed pods in autumn - after they've turned brown - and carefully open the seed pod/bean length-wise. Do not remove the seeds from the pod. Having the seed pod partially open will help the seeds sprout more easily in spring.
Take the pod and plant it in a pot that is large enough for it to fit in. Cover the pod with a half-inch of soil. Leave the pot outdoors throughout the autumn and winter season to expose it to the cold (cold stratification). You can also plant the seed pod in the ground at a half an inch down.
In spring, you should begin to see sprouts popping up. When they're about an inch or two tall, carefully separate the sprouts, keeping the healthiest ones, and transplant them where you want your tree(s) to be.