Is this a new system ? What Model/ONTAP? Ideally, Just 3 disks are enough for the root_aggregate. Looks like more disks were added to increase the root aggregate ? But, it serves no purpose. The root aggregate is dedicated to the node’s root volume only. ONTAP prevents you from creating other volumes in the root aggregate.
You will need to re-initialize the system to use the correct set of disks. That would leave '7' spare disks, you can use '5' to create data_aggr and 2 you can keep as spare.
If you wish you can go for ADP: This will enable you to extract more space from single disk by using partitioning mechanism.
ADP: With Advance Disk Partitioning (ADP), a physical disk is divided into a small number of partitions at the storage level, and each partition is treated by RAID and HA as a logical disk. Aggregates are provisioned from partitions, instead of from whole disks.