About NodeJS, it is a JavaScript server-side language, who is pretty and quite easy to get started with, but I don't think there is a special interest for an autocomplete feature ?!
I don't exactly know how tagit works, but if you want to use bootstrap to manage it, you can easily use the "typeahead" plugin and set the items option to the max number of items you wants.
http://twitter.github.io/bootstrap/javascript.html#typeahead
$('#demo2').typeahead({
source: availableTags,
items: 10
});
EDIT: according to the tagit.js documentation, and the jQuery autocomplete one.
https://github.com/aehlke/tag-it#autocomplete-object
Allows overriding the source and select options that are set by
default, as well as adding any other options you want to pass to the
jQuery UI Autocomplete widget, such as minLength or delay.
The autocomplete.source should be overridden if you want to use custom
autocompletion sources, like an Ajax / XHR response.
http://api.jqueryui.com/autocomplete/
You cannot easily add an option to limit the max number of items displayed, however you could overwrite the source with a function where you can limit the response items inside !
I keep thinking that bootstrap is more adapted to your needs, and it doesn't require jQuery-UI.
EDIT:
After discussion, this plugin should probably be the one you are looking for !
http://welldonethings.com/tags/manager
$('#demo2').tagsManager({
typeahead: true,
typeaheadSource: availableTags
});