If these are just standard Jalapenos then you can probably find more plants at a nursery. Note that different Jalapeno varieties are out there with different levels of heat, so don't throw away the tags.
I would absolutely stick at least 9 plants in there. Most pepper plants do not need more than 1 square foot of space.
Although actually I've had the best results with containers. One more thing, if you want those peppers HOT, then hold back on watering the 4-5 days before harvest. I had a Habanero plant and let's just say I didn't treat it as nicely as I should have. Those peppers were hotter than lava.