Variable: prefix-arg

The value of the prefix argument for the next editing command.
It may be a number, or the symbol `-' for just a minus sign as arg,
or a list whose car is a number for just one or more C-u's
or nil if no argument has been specified.

You cannot examine this variable to find the argument for this command
since it has been set to nil by the time you can look.
Instead, you should use the variable `current-prefix-arg', although
normally commands can get this prefix argument with (interactive "P").