How do I find the inode of any directory?

Yes, the argument -i will print the inode number of each file or directory the ls command is listing. As you want to print the inode number of a directory, I would suggest using the argument -d to only list directories. For printing the inode number the directory /path/to/dir, use the following command line:

ls -id /path/to/dir

From man ls:

   -d, --directory
          list  directory entries instead of contents, and do not derefer‐
          ence symbolic links
   -i, --inode
          print the index number of each file

This works also with stat:

DIR=/
stat -c '%i' $DIR

From man stat:

   -c  --format=FORMAT
          use the specified FORMAT instead of the default; output  a  new‐
          line after each use of FORMAT
[...]

   The valid format sequences for files:    
       %i     inode number

You can find inode of file and directory with -i option

ls -id /home/user/dir

You can get information of consumed inode of your system

df -hi