You can reserve only 7 nights if booking exactly 11 months out from date of arrival for home resort and 7 months out for other resorts, regardless of whether you do it online or by phone. You can call MS the next day and add an eighth night, or, alternatively, do not try to reserva any days until that second day when you can reserve all 8 nights at once.
Note there is no limit to the number of nights you can reserve except for points available, e.g., at ten months out you can reserve a month at one time. The way to look at the actual rule is that your departure date from WDW can never be more than 11 month plus 7 days (home resort), or 7 months plus 7 days (non-home resort), from the date you are trying to make a reservation.
