The maximum number of columns per index is 16. The maximum key length
is 1000 bytes. This can also be changed by changing the source and
recompiling. For the case of a key longer than 250 bytes, a larger key
block size than the default of 1024 bytes is used.
The InnoDB internal maximum key length is 3500 bytes, but MySQL itself
restricts this to 3072 bytes. This limit applies to the length of the
combined index key in a multi-column index.