It's the section number, see
A section, if provided, will direct man to look only in that section
of the manual. The default action is to search in all of the
available sections, following a pre-
defined order and to show only the first page found, even if page exists in several sections.
The table below shows the section numbers of the manual followed by the types of pages they contain.
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
stat have 3 sections :
$ man -k stat | grep "^stat "
stat (1) - display file or file system status
stat (2) - get file status
stat (3p) - get file status
So if you type
man 1 stat
it's not the same as
man 2 stat