As a general rule of thumb: Cryptonight-light, the PoW hash AEON uses, needs a 1 MB scratchpad in fast memory, so you will want to have all your mining threads able to keep their scratchpad in cache. So the optimal number is likely to be your cache size divided by 1 MB (eg, if your L3 cache size is 4 MB, then 4 threads). If that number happens to be higher of equal to the number of cores you have, then keep it to the number of cores minus one.
If you use the machine for other things at the same time, you will need to decrease that number, and by how much will have to be determined by testing.
In any case, you may want to test a number of configurations, testing is quick and easy, and will uncover any deviation your hardware may have from that rule of thumb.
The one and only thehihoguy.
Poolowner of Serenity and maintainer of Hidden Jadestone.
Modesty in person:P