A minimum of 2 HDDs is required to create a RAID 1.
The data is simply cloned on each HDD of the RAID 1.
This RAID system is very secure, but isn't fast with reading/writing. Indeed, the data must be written on each HDD of the RAID 1 everytime.
The only thing with this RAID system is that you'll be able to use the whole volume of a single HDD. As each other HDD is a clone of the first one, you'll be able to use only one of them to store your data.
The common RAID 1 failures are :