Command for toggling line numbers in vi

You can use (on VIM at least):

:set invnumber

More Info:

:set number      Turn line numbers on
:set nonumber    Turn line numbers off
:set invnumber   Toggle line numbers
:set number!     Toggle line numbers
:set number&     Set option to default value
:set number?     Show value of option 

source: http://vim.wikia.com/wiki/Managing_set_options#Boolean_options