Cryptonight, the Proof-of-Work (PoW) algorithm used by Monero, uses a 2 MB buffer to do its calculations. Mining speed is heavily influenced by whether this buffer can fit in your CPU's cache. If it can't, some reads and writes will hit RAM, and slow it down. Therefore, to determine the optimal number of mining threads (at least as far as cache is concerned), you work out how many such buffers can fit in your cache. If your cache is 4 MB, then 2 buffers can fit. This is where the "divide by 2" comes from. Note that if you use your computer at the same time as mining, you may want to go with fewer threads, so your normal workload does not get slowed down.
So, you'll have to check your system's specs to see how much cache it has. It's probably between 2 and 6 MB, giving 1 to 3 threads. In any case, it's safe to experiment with small number of threads, and see how your hash rate varies, then use the setting which seems to get you the best performance.
Note that mining using the GUI like this is called solo mining, as opposed to pool mining. In the long run, solo and pool mining give about the same expected mining gains. However, solo mining will be very much more unpredictable in the timing of the gains. It is like a lottery: you can play for a very long time without winning, but when you win, you win BIG. With pool mining, you win often, but only tiny sums. In the long run, it works out about equal, but make sure you are OK with this before choosing between pool and solo mining. Note that solo mining is also better for the network as a whole, as it avoids centralizing mining to the pools.