And it depends on where you make the reservations as to how many people it takes to make an automatic gratuity.
This one is fairly clear on TS dining at Disney - a party of 6 or more connotates an automatic gratuity of 18%. It doesn't matter if it's three adults, one child & two babies, or six adults - once you have an ADR for six, you're hit with the automatic gratuity - at least at the Disney-run restaurants. It's stated on all of the menus and on the cards they put in the bill folios. This is from a previous post on the discussion - you can run a search for more information:
Direct from the 2009 DDP brochure:
Q. Are gratuities included?
A. No, gratuities are not included unless otherwise indicated. An 18% gratuity will
be added automatically for parties of 6 or more. A gratuity may also be added
automatically for ordered items that are not included in the Disney Dining Plan (e.g. alcoholic beverages).
Now, as far as making ressies for a party of six on your confirmation number - since you're making the reservation, you can do it without any problems - you're a "party". Be cognizant, however, that the larger the party the harder it is for Disney to accommodate you due to the party size.