What value should kernel parameter AIO-MAX-NR be set to ?

The minimum value suggested as per https://docs.oracle.com/database/121/LTDQI/toc.htm#BHCCADGD for aio-max-nr is 1048576 .
We recommend setting aio-max-nr to 3145728 in case that ORA-27090 occurred.
And the maximum of aio-max-nr parameter can be calculated by -
aio-max-nr =no of process per DB no of databases 4096
After deriving the value using this formula , you should still monitor the /proc/sys/fs/aio-nr value and also monitor I/O loading.
Points to know -
1) Number of processes per database is not background server process number, it's the number of background server process + user
process.
2) Parameter aio-max-nr is a limitation of aio-nr, no impact was reported if you set it too high. But we should still monitor /proc/sys/fs/aio-nr
value and tune this value.