Each additional data disk you add will be additional data capacity. If you add another parity disk (up to 2 are allowed) it won't give any additional storage, but it will provide additional redundancy.
Each parity disk must be at least as large as any single data disk. So, you can add a lot more disks, each up to 14TB, and they will have parity protection from that single parity disk. Single parity allows recovery of a single data disk, dual parity allows recovery of 2 data disks.
Take a look at this wiki overview for a better idea of how parity works, how user shares work, and many other things about Unraid features:
https://wiki.unraid.net/UnRAID_6/Overview