Rorate Caeli reports that there were, after all, thirteen signatories to The Letter. They were, according to Rorate:

• Carlo Cardinal Caffarra, archbishop of Bologna*
• Thomas Cardinal Collins, archbishop of Toronto
• Daniel Cardinal DiNardo, archbishop of Galveston-Houston
• Timothy Cardinal Dolan, archbishop of New York*
• Willem Cardinal Eijk, archbishop of Utrecht
• Gerhard Ludwig Cardinal Müller, prefect of the Congregation for the Doctrine of the Faith*
• Wilfrid Fox Cardinal Napier, archbishop of Durban*
• John Cardinal Njue, archbishop of Nairobi
• George Cardinal Pell, prefect of the Secretariat for the Economy*
• Norberto Cardinal Rivera, archbishop of Mexico City
• Robert Cardinal Sarah, prefect of the Congregation for Divine Worship*
• Elio Cardinal Sgreccia, president emeritus of the Pontifical Council for Life*
• Jorge Cardinal Urosa, archbishop of Caracas

Rorate does not report its source for this new information [SEE EDIT], and, while Sandro Magister has removed the names of the cardinals who disassociated themselves from the letter, he has not updated his blog to reflect the new purported signatories. Obviously, if there is a new round of denials, corrections, or dissociations, we will report it.

We have marked with an asterisk those prelates who are present at the Synod ex officio as dicastery heads (subject to papal appointment) or by special appointment of the Pope. Of the thirteen, we count seven cardinals on this list who are at the Synod essentially as papal nominees.

EDIT: Rorate did apparently report its source: Gerard O’Connell at America. O’Connell said,

America has learned from informed sources that 13 cardinals did indeed sign the letter, including four not named on Magister’s list:  Di Nardo (United States), Njue (Kenya), Rivera (Mexico) and Sgreccia (Italy). The full list of signatories is given below.

(Italics supplied.)