Don't stress about it so much.
A Battery cycle refers to, more or less, one mostly full discharge and recharge. Typically, your battery should be rated for somewhere in the neighborhood of 1000 cycles. Eventually, the battery just won't hold a charge for as long as it did when it was new, but that's why batteries are easily and affordably replaceable. It's a consumable part. If it dies prematurely (which is the only thing you should really need to pay attention to cycles for), then it's covered under your warranty. The 'Battery Cycles' indicator is mainly there so that you can diagnose this. If, in a years time, your laptop's battery only lasts half as long as it did when it was new, but you've only used say, 400 cycles, well, that's not performing up to spec, and it should be covered under your warranty. If you've used 1500 cycles (doing that in a year would be some pretty heavy and impressive use though, I've gotta say), well, that's to be expected. You used your battery a lot, and it wore out.
Use your laptop as normal. Battery cycles are not a scarce and precious resource to be hoarded. 12 cycles in about a week and a half sounds pretty normal for a new laptop being used heavily to me. That's about, what? 6 hours/day unconnected to a power source? If you're using it on the go, that's a perfectly reasonable amount of consumption.
Unfortunately, Target Display Mode doesn't work on MacBook Pros. However, this thread here describes various ways to use the MacBook Pro as a screen. In a nutshell, it says to use a third party app such as Air Display or Screen Recycler (both of which, it appears, operate over a network. The thread suggests connecting the two computers with a thunderbolt cable and thunderbolt network to speed up the connection. For more details, see the link above.
Best Answer
A charge cycle is just the process of using and then recharging the full capacity of the battery one time. From Apple:
You can't really have "too many" charge cycles unless there's something wrong with the battery and it's unable to hold a charge. That seems unlikely since your computer is brand new (and OS X will notify you if your battery condition is abnormal).
Furthermore, your particular model is rated for 1,000 charge cycles before it's considered "consumed" (meaning it'll no longer hold a reasonable charge and will likely need to be replaced). If your current rate of 13 charge cycles per three weeks holds, that means you're good for 4.5 years – in other words, nothing to worry about!
Even if you leave your laptop plugged in at 100%, the system will actually silently discharge the battery to ~95% and back up to keep things moving. Apple used to recommend unplugging your laptop at least once a month, but their current guidelines make no mention of that, so I wouldn't worry about it.