Hoopster summed it up quite well, but I wanted to stick my .02 into the discussion to hopefully clear this up a little more.
Parity doesn't hold any data. Period. It's not a backup. Period.
It contains the missing bit in the equation formed by adding up the bits in an address row. Pick any arbitrary data offset, say drive1 has a 0, drive2 has a 1, drive3 has a 1, drive4 has a 1, so parity would need to be a 1 to make the column add up to 0. Remove any SINGLE drive, and do the math to make the equation 0 again, and you know what bit belongs in that column of the missing drive.
So, you can protect ANY number of drives, and as long as you only lose 1 drive, the rest of the drives PLUS PARITY can recreate that ONE missing drive. Lose 2 drives, and you lose the content of both, but since Unraid doesn't stripe across drives, you only lose the failed drives.
Unraid has the capability to use two parity drives, so you can recover from 2 simultaneous failures. However, the second parity is a much more complex math equation that takes into account which position the drives are in, so it's a little more computationally intensive. The extra math is trivial for most all modern processors.