Linux functions, why do they have number inside?

Question

for example execve(2) is the function of execve login(1) is the function of login, what does the number means?

Answer 1

Those are the manual section numbers. From the man manual:

 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, e.g. /etc/passwd
   6   Games
   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]

Answer 2

The number refers to the section of the man-page.

Some (most?) topics have entries in multiple sections, so the section number is used to differentiate them. Foe example, there's a topic for login in the User Commands section (i.e., login(1)) and in the Linux Programmer's Manual (i.e., login(3)).