Muhammad Ibn Ali As-Saduq writes:

قَـالَـتِ الـزَّيْـدِيَّـةُ لَا يَـجُـوزُ أَنْ يَـكُـونَ مِـنْ قَـوْلِ الْأَنْـبِـيَـاءِ إِنَّ الْأَئِـمَّـةَ اثْـنَـا عَـشَـرَ لِأَنَّ الْـحُـجَّـةَ بَـاقِـيَـةٌ عَـلَـى هَـذِهِ الْأُمَّـةِ إِلَـى يَـوْمِ الْـقِـيَـامَـةِ وَالْاِثْـنَـا عَـشَـرَ بَـعْـدَ مُـحَـمَّـدٍ صَـلَّـى الـلَّـهُ عَـلَـيْـهِ وَآلِـهِ قَـدْ مَـضَـى مِـنْـهُـمْ أَحَـدَ عَـشَـرَ وَقَـدْ زَعَـمَـتِ الْإِمَـامِـيَّـةُ أَنَّ الْأَرْضَ لَا تَـخْـلُـو مِـنْ حُـجَّـةٍ فَـيُـقَـالُ لَـهُـمْ إِنَّ عَـدَدَ الْأَئِـمَّـةِ عَـلَـيْـهِـمُ الـسَّـلَامُ اثْـنَـا عَـشَـرَ وَالـثَّـانِـيَ عَـشَـرَ هُـوَ الَّـذِي يَـمْـلَأُ الْأَرْضَ قِـسْـطًـا وَعَـدْلًا ثُـمَّ يَـكُـونُ بَـعْـدَهُ مَـا يَـذْكُـرُهُ مِـنْ كَـوْنِ إِمَـامٍ بَـعْـدَهُ أَوْ قِـيَـامِ الْـقِـيَـامَـةِ وَلَـسْـنَـا مُـسْـتَـعْـبَـدِيـنَ فِـي ذَلِـكَ إِلَّا بِـالْإِقْـرَارِ بِـاثْـنَـيْ عَـشَـرَ إِمَـامًـا اعْـتِـقَـادَ كَـوْنِ مَـا يَـذْكُـرُهُ الـثَّـانِـيَ عَـشَـرَ عَـلَـيْـهِ الـسَّـلَامُ بَـعْـدَهُ

“The Zaidites say: “It cannot possibly be the statement of the prophets that the Imams are twelve, for the proof remains until the day of resurrection, and of the twelve after Muhammad (s.) eleven have died and the Imamites have claimed that the earth is not free of one Imam.” So they are told that the number of Imams (a.) is twelve and the twelfth is the one who fills the earth with justice and equality. There will be after him what he mentions, namely one Imam after him or the coming of the resurrection, and we submit ourselves in this to the confession of twelve Imams only, with the belief that there will be what the twelfth (a.) mentions after him.” [Kamal-ud-Din, page 82]

Comment:

Muhammad At-Tusi considers more than twelve imams to be impossible [Here!]