raid 10 how many drives can fail ?

 not funny in a QNAP with 2 drives fail

depends on which 2 drives fail

https://www.computerweekly.com/answer/Recovering-from-a-two-drive-failure-in-a-RAID-10-configuration

https://www.promax.com/blog/how-many-drives-can-fail-in-a-raid-configuration

RAID 10: This RAID can survive a single drive failure per array.