What's the average number of spare sectors use to replace defective
sectors aka bad sectors in hard drives nowsaday? And what I means with
nowadays are harddrives with capacity of 120GB or above. Is there an
average number of spare sectors per cylinders/tracks across
manufacturer?

Spare sectors (bad sector replacements) are physical sectors that
occupy space at the end of the last track in the cylinder.  They are
deducted from the sectors/track of the last track of each cylinder
since they are not available to the file system for data allocation.

I hope it helps.
Phossil

I knew that already, but I also heard that there are also spares per
cylinder. And just really curious about how many of them, not as much
as where are they. Thanks for the info anyway.

Since engineering of hard drives has been vastly improved since the
late 80's when spares were popular, most hard drives don't have
spares.  Spares have been proven to drastically slow down a computer -
most modern OS file system tables, drivers, and cache managers are
capable of managing bad clusters better than the hard drive controller
can.

So, the answer is zero to the "how many" question and nowhere to the
"where" question.