the wear on the other tire on that axle and even the wear on the tires on the other axle.
And even the amount of water-on-the-road driving you do.
The traction on each end of an axle should be about equal. So you don't want a new tire on the same axle with a tire down to the wear bars. The general rule of thumb is "At least same tread depth and same tread design on an axle". So what is "same depth"? With 70% of original tread depth or more it is safe to replace just one tire. Get much below that and you are violating the "same depth" rule and it starts to get dangerous because you are creating different traction on each side of the car and a different ability to evacuate water away from the contact patch.
By keeping the tread depth nearly the same and the tread pattern the same, you are maximizing the effectiveness of anti-slip brakes and Porsche's Stability Management (PSM).
If you do get just one tire, watch the wear on the older tire and don't let it wear down to where it has no tread depth just because you are trying to get maximum use out of the replacement tire. Remember tread depth differences can cause accidents.
Accidents hurt ...your pocketbook ... your insurance rating ... your health...someone else.
Consider if it is better to replace 4 tires so they all are new, match tread patterns, match tread depth..and use tires that are still Ultra High Performance Summer Radials but just aren't as super capable or super priced as some others. Kumho and Sumitomo are less expensive brands often recommended.
If you have Internet access...go look at the
www.tirerack.com site for a list of tires that work on your car in your sizes and see. Often retailers will match prices or come close enough.