How to pluralize "There is/are N object/objects"?

This is probably best covered by the Rails i18n pluralization features. Adapted from http://guides.rubyonrails.org/i18n.html#pluralization

I18n.backend.store_translations :en, :user_msg => {
  :one => 'There is 1 user',
  :other => 'There are %{count} users'
}
I18n.translate :user_msg, :count => 2
# => 'There are 2 users'

You can add a custom inflection for it. By default, Rails will add an inflections.rb to config/initializers. There you can add:

ActiveSupport::Inflector.inflections do |inflect|
  inflect.irregular "is", "are"
end

You will then be able to use pluralize(@total_users, "is") to return is/are using the same rules as user/users.

EDIT: You clarified the question on how to pluralize a sentence. This is much more difficult to do generically, but if you want to do it, you'll have to dive into NLP.

As the comment suggests, you could do something with I18n if you just want to do it with a few sentences, you could build something like this:

  def pluralize_sentence(count, i18n_id, plural_i18n_id = nil)
    if count == 1
      I18n.t(i18n_id, :count => count)
    else
      I18n.t(plural_i18n_id || (i18n_id + "_plural"), :count => count)
    end
  end

  pluralize_sentence(@total_users, "user_count")

And in config/locales/en.yml:

  en:
    user_count: "There is %{count} user."
    user_count_plural: "There are %{count} users."