How many parity disks do I need?

[AngrysDad]

Hi everyone, Total Noob here. I've had my unraid server going for a total of about 8 hours now. I'm currently transferring data from my old Drobo onto my newly built unraid rig.
Currently, I have 4 brand new 4Tb HDD in my array with 2 data disks and 2 parity disks, with a 1Tb Samsung Evo 970 plus as Cache. This gives me just under 8Tb of storage space with dual parity (I think - correct me if I'm wrong).
I would like to repurpose 2 of the HDD's from my old Drobo in the new unraid rig. These are a nearly new 4TB Western Digital Red, and a generic 5Tb Samsung HDD.
My question is: Is it possible to add these disks into the system and increase my storage space and still retain dual parity (ie 2 disk failures without losing data)?
If this is possible, how do I assign each disk to set this up?
I hope someone out there takes pity on a noob. I thank you emphatically for any help that anyone can offer.

[itimpi]

There should basically be little problem assuming you have sufficient SATA ports to have all drives connected and you are within your Unraid licence limits (the 3 new drives takes you to 8).     The only caveat is that since no data drive can be larger than the smallest parity drive you would need to make the 5TB drive a parity drive (releasing the 4TB it replaces).  
 

This section of the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page will probably help.  The Unraid OS->Manual section in particular covers most features of the current Unraid release.