PSU chamber - 8 slots (3.5" only)
Motherboard chamber - 2 slots (compatible with both 2.5" and 3.5")
Behind the front panel - 2 slots (2.5" only)
So, there are 12 slots in total, where you can place a maximum of 10 x 3.5" HDD's or a maximum of 4 x 2.5" HDD's.