Why max inode count became 2^32-1 on my hdd after update to OS X 10.12 Sierra
You do not detail which file system you're using, so I'm assuming you're using the default HFS+ file system on Sierra.

The HFS+ file system does not have an "inodes" concept similar to other file systems. Therefore the inodes free/used counts really do not make sense for HFS+, and never have.

In order to comply with standards the df command on macOS do report inode information. However it is just made up figures more or less.

On older version of macOS the df command consistently reported that you were using approx. the same ratio of inodes as the ratio between free and used space. So if you were using 500 GB out of a 1 TB drive, it would report that you've used about 50% of your inodes. 

On Sierra Apple must have decided that it was better to just report 0% usage. Either number is okay as it is entirely bogus anyhow.

In the underlying file system on disk, nothing have changes in regards to the maximum number of files you can save. It was (2^32)-1 before, and it is still (2^32)-1 now.