Hi Milind,
i dont know about the max limitation of the username but 5 username is not an issue.
you can configure the usename with this command.
>enable
#
# config terminal
# usename ***** password ***
or for encry
username ***** secret 0 to 5 *******
0-5 is level type any one.
keep using same command for all users
no delet them say no and command.
no username ****** password *****
Dinesh