Iron Cross Oxalis

How can I get these bulbs to increase so I can have a larger plant?

Most Oxalis increase readily on their own if they're growing happily. They tend to be very forgiving plants, so standard care (good potting media, proper light, regular feedings) should achieve your goal.

Do they increase by making more bulbs? Do I need to let them fully flower so they will make more bulbs?

Thanks

Yes, lots of baby bulbs are produced under the soil. I don't recall seeing seed heads on my Iron Cross, but never really looked for them either.