Minimum disks needed for RAID 10 is 4.
When using RAID 10, 2 RAID 1 volumes are created and then these 2 volumes are put in RAID 0 mode.
Do not confuse this with RAID 01, which creates 2 RAID 0 volumes and then mirrors them.
Linux MD RAID10 is a non-standard level that can permits setting up unusual layouts beyond what is possible with the standard RAID10. MD RAID10 requires a minimum of only two drives.