This will depend on the motherboard used and type of power supply (i.e. the ZF240 or Nano). Below is table to show how many 3.5″ drives can be fitted depending on the combination for hardware

 

Motherboard ATX M-ATX M-ITX
NanoPSU 0 x 3.5″ 1 x 3.5″ 3 x 3.5″
ZF240 0 x 3.5″ 0 x 3.5″ 2 x 3.5″