An inode number is an integer.
The #/inodes in a filesystem is finite. If you exceed the #/inodes allowed, you cannot create any more files. You must first increase the size of the inode table.
The #/inodes in a filesystem is determined *only* by the capacity of the inode table, *not* by the numeric range of the inode number. Inodes are *not* susceptible to "integer overflow" (like file size could be).
Here's more info: