Without limitation with external NAS and RAIDs; you can put 2-n numbers of drives in any RAID configuration. The down side is that the RAID controller can handle and expensive to setup. Internally, it's all up to what the power supply can handle and the physical limitation of the case enclosure. If you have a 650W or better PS, you should have no problem putting 7-9 drives in a full tower. I did 19 at one time with the Thermaltake 750 and no a problem, but the cheap PS may give you a power drain issue. In that case, I guess you will find out one way or the other when your system flickers.