A single copy license of Photoshop is either Mac or PC. You'd need to buy Photoshop again to switch to Mac. The only answer I can tink of is the Creative Cloud license which is cross platform.
I am wanting to buy the photoshop cs6 and am currently running Microsoft but want to upgrade my laptop by the end of the year to a Mac...
You can do a "Crossgrade" to change platforms, the information on the process is here, please make sure you read it carefully and understand all the steps. If you have any questions about the process, or are unsure about any of the steps, contact Adobe Customer Service directly, they are the only ones who can give you absolute answers in areas such as this.
As for which Mac to buy, I work in IT sales so I'll give you a trite "sales" answer, but it's advice I follow myself when I build my own systems; simply buy the most powerful system you can with the budget you have available on the day!
I assume from the way you've phrased your question that you're not interested in a desktop? Okay, the 17" MacBook Pro is a fantastic machine, but it's a beast to haul around, so if you need to be "mobile" you may want to consider a maxed-out 15" machine, plus a decent - 24" 1920x1200, IPS panel - monitor for when you're working at your desk.
A good place to start would be to visit Apple and play with the configuration tools for the 15" and 17" machines, plug in the most powerful processors, and fastest hard drives, then see if the $'s are in your budget. A word of advice, do not buy additional RAM from Apple, the prices are outrageous and cannot be justified, buy the machine with the minimum amount of RAM available, then purchase good 3rd party RAM such as Kingston - no, I don't work for Kingston! - and install it yourself, it's easy. In fact, once you've spec'd out a machine, shop it 'round Apple's authorized resellers, I'll pretty much guarantee you'll get a better deal.
Can you only use the keycode once when you purchase it?
In literal answer to your question, if you were to want to use Photoshop on two different computers of the SAME variety (e.g., both PC, no Mac), you can use your serial number to activate Photoshop up to two times.
Read the EULA for the specifics.