How many drives do i need?

[Uncontrolled Competence]

I am targeting a 20TB NAS but that has expand ability in the future. How many drives do i need for this and what raid should i run it at for the best protection?
FYI that 20TB is after the raid has taken some drives for redundancy.

[cake]

How much redundancy do you want, meaning, how many drives do you want to be able to fail before you start losing data? If you want to allow one drive to fail without losing data, then you'll need 2, if you want to allow 2 drives to fail without losing, you'll need 3.

These are called Raid-Z1 and Raid-Z2 respectively. You can use this calculator: https://wintelguy.com/zfs-calc.pl

[Uncontrolled Competence]

OK thanks @cake for the help i have decided to go for a raid-z2 in a pool size of 5 drives. I havnt yet bought the drives so my post was about what i would need to reach that goal of 20TB before i went out and bought the drives but the calculator was still very helpful.