Anyone know why my 300C has 16 sparkplugs and why the recommended change interval is only 30,000 miles?
I am guessing Chrysler is trying to pull a ton of power out of that engine and is pushing it hard.
You went for the power and now you pay. 
I’ll bet that it does not do outstanding in the mileage department.
While two spark plugs per cylinder is a bit unusual (even though Nash featured that on some of their engines 60-70 years ago), a change interval of 30,000 miles is not unusual with modern platinum or iridium spark plugs. In fact, that is a pretty typical change interval for those plugs–and on many engines the recommended change interval is as long as 60k.
The problem with leaving spark plugs in place for such an extended period of time is that they can become virtually impossible to remove when it is time to change them. Personally, I would not go any longer than 30k before changing my spark plugs, and if you feel comfortable doing it sooner, feel free to do so. However, you might want to check out the cost of 16 platinum or iridium plugs before you decide to change them. Just be that you are in a seated position when you are told the price!
I imagine Chrysler used 16 plugs in the engine (it’s a Hemi, right?) is so that there will be equal flame front distribution, and therefore more power.
It has 16 sparkplugs because you had to have a HEMI.
It uses one plug (the one directly under the coil pack) on the power stroke, and one plug (the one attached to the plug wire) on the exhaust stroke to reduce emmisions. The largest reason for the 30,000 mile change interval is to meet emmision requirments. Although you will not notice the plugs start to break down at about 30k. You probably will not notice any problems until about 50k when the check engine light comes on due to intermitent misfire. Due to the fact that manufactures are required to cover emmision devices for up to 8 years and 80,000 miles they don’t want them damaged due to lack of maintenance.