Propagating Cactus. Their are 2 main ways to propagate cactus: from seeds, and from cuttings. Below is a tray of seedlings that are ready to be seperated and potted into indivudual containers. Drawbacks to growing by seed are the time it takes to grow from seed to seedling, to transfer ready. Pros are you can grow a lot in a small area, and can purchase seeds from all over the country/world etc.
The other option is to take cuttings from cactus, allow them to dry/callous, and then replant into containers. Generally you will get new growth from the cut areas as well so you have a cycle of cactus to cut from, limited only by their growth time. This way takes more space though but growth is quicker as the parent cacti have established roots and more mature size.
However you choose to propagate your cacti, have fun doing it and multiply those guys with success!