you wont encounter any limitation regarding the amount of drives or storage space when you use FlexRAID. it allows you to add as many data drives as you want. and you can also use as many parity drives as you want (1 parity would make it equal to raid 5, 2 parity would make it equal to raid 6)
i take it you mean 6*2TB drive? if so then i would use 1 parity drive, so that way you will still have 10TB left (but keep in mind that 10TB would be 9.3TiB use this converter if you need to check http://www.unitjuggler.com/convert-memory-from-TB-to-TiB.html)
depends on the amount of drives you wish to use, i would pick 1 parity drive per 7 data drives. so 7 drives and 1 parity, and more then 7 i would use 2 parity drives (personally have 14 data drives and 2 parity drives)
yes, the motherboard has nothing to do with the storage in this case, all the board needs to do is make the drives show up as usual in the OS
well, if i would build a system now then i would pick a Intel i3 or i5 and 8 or 16GB of ram, and a motherboard that offers expandability when i comes to PCI-e slots (that's what the raid cards use) and i would use a cheap raid-card or host bus adapter. like the IBM m1015, that would allow you to add 8 more drives to your server.but the sata ports on your motherboard might suffice for your current needs.
hope this answered your current questions, im sure you will have more so keep them coming and we will see about answering them.
looney
