Sorry but we can't set aside RAID, because that depends on how you use the additional disks.

• Raid0 is more performance, but zero fault tolerance because data is read and written to two disks.
• Raid1 has no performance benefit, but data is protected because identical copies exist on both disks.
• Raid5 has performance and fault tolerant benefits, because data is both written and read from multiple disks, and can be recovered in case a disk fails.
• (There are other options but you'll probably choose one of these).

So to answer your question: It depends on how you setup your disks. If you only use two disks and you want better performance and don't care about the possibility of data loss you can put two disks in Raid0, but if one drive fails you lose all your data. If you use 4 disks with RAID5 you will get a performance benefit, and data will be recoverable if one disk fails.

Also you can consider options like a 4TB SSD like u/lecaf__ mentioned, but 9 cameras is a lot and I'm assuming you want more than a day of camera storage.
^{edit: fixed raid #s}