- You are probably best off hitting dat <kbd>Tab</kbd> key 5 times in your example.

- More elegantly, you can have vim repeat the typing for you. But with its ≥7 keystrokes, this technique is only economical for a bigger number of repetitions or characters to be repeated:  
`:%s/\s\(item.\)/\r`     <kbd>Ctrl</kbd>-<kbd>f</kbd> <kbd>`5`</kbd><kbd>a</kbd> `\t` <kbd>Ctrl</kbd>-<kbd>c</kbd>     `\1/g`

- At the cost of more verbose syntax, the technical answer to your question is using the `\=` syntax element for substituting by an expression [[`:help sub-replace-expression`](http://vimhelp.appspot.com/change.txt.html#sub-replace-expression)], in which you could then employ the [`repeat()`](http://vimhelp.appspot.com/eval.txt.html#repeat()) function:    
`:%s/\s\(item.\)/\= "\n" . repeat("\t",5) . submatch(1)/g`  <sub>  (Readability spaces not required.)</sub>

Unfortunately, vim does not provide syntactic sugar for repetition (`\{5}` or otherwise) in the replacement part of the `:substitute` command;  almost none of the special regex syntax for pattern matching [[`:h pattern-overview`](http://vimhelp.appspot.com/pattern.txt.html#pattern-overview)] is available for the replacement: [`:h sub-replace-special`](http://vimhelp.appspot.com/change.txt.html#sub-replace-special)