First it's good to understand that thee devices use different types of energy. Fraxel Restore is a laser and eMatrix uses radio frequency.
A laser has the potential to create hyper-pigmentation on certain skin types as it passes through the pigment-producing cells and can activate them, so it's generally reserved for those with light skin and more Nordic backgrounds. Gene pools with darker skin pigment run a higher risk for post treatment pigmentation with laser.
eMatrix "bypasses" the pigment producing cells (for the most part), making it friendlier to skin that has a tendency to discolor.
As to number of treatments, it's individual and will depend on several factors such as
Density of your pigmentation
Cause of pigmentation
Your skin's unique response
Your adherence to pre and post treatment advice
But think more in terms of 3 to 4 or possibly more treatments - It is not always possible to completely remove pigmentation.
No matter which treatment you choose - or additional treatments you add to the mix - sunscreen and heat avoidance will help maintain the results.