Muhammad Ibn Hasan At-Tusi writes:

أَمَّـا غَـيْـبَـةُ عِـيـسَـى عَـلَـيْـهِ الـسَّـلَامُ فَـإِنَّ الْـيَـهُـودَ وَالـنَّـصَـارَى اتَّـفَـقَـتْ عَـلَـى أَنَّـهُ قُـتِـلَ فَـكَـذَّبَـهُـمُ الـلَّـهُ عَـزَّ وَجَـلَّ بِـقَـوْلِـهِ وَمَـا قَـتَـلُـوهُ وَمَـا صَـلَـبُـوهُ وَلَـكِـنْ شُـبِّـهَ لَـهُـمْ كَـذَلِـكَ غَـيْـبَـةُ الْـقَـائِـمِ فَـإِنَّ الْأُمَّـةَ سَـتُـنْـكِـرُهَـا لِـطُـوْلِـهَـا فَـمِـنْ قَـائِـلٍ يَـقُـولُ إِنَّـهُ لَـمْ يُـولَـدْ وَقَـائِـلٍ يَـفْـتَـرِي بِـقَـوْلِـهِ إِنَّـهُ وُلِـدَ وَمَـاتَ وَقَـائِـلٍ يَـكْـفُـرُ بِـقَـوْلِـهِ إِنَّ حَـادِيَ عَـشَـرَنَـا كَـانَ عَـقِـيـمًا وَقَـائِـلٍ يَـمْـرُقُ بِـقَـوْلِـهِ إِنَّـهُ يَـتَـعَـدَّى إِلَـى ثَـالِـثَ عَـشَـرَ فَـصَـاعِـدًا وَقَـائِـلٍ يَـعْـصِـي الـلَّـهُ بِـدَعْـوَاهُ إِنَّ رُوحَ الْـقَـائِـمِ عَـلَـيْـهِ الـسَّـلَام يَـنْـطِـقُ فِـي هَـيْـكَـلِ غَـيْـرِهِ

“As for the concealment of Jesus (a.), the Jews and Nazarenes agreed that he was killed, whereupon God declared them liars by His Word: “Neither did they kill nor crucify him, but it appeared so to them”(4:157) Such is the case with the concealment of the Riser (Qa’im), for the nation will deny it because of its long duration. Thus there is a proponent who says: “He was not born.” And there is a proponent who makes use of the lie by saying: “He was born and then died.” And there is a proponent who, by his statement, falls prey to disbelief: “Our eleventh was barren.” And a proponent who falls away by his statement: “It extends to a thirteenth. Then rising.” And a supporter who disregards God by his assertion: “The spirit of the Riser (Qa’im) talks in the shell of another.” [Al-Ghaibah, page 111]

فَـأَمَّـا مَـنْ قَـالَ إِنَّ لِـلْـخَـلَـفِ (الْـقَـائِـمِ) وَلَـدًا وَأَنَّ الْأَئِـمَّـةَ ثَـلَاثَـةَ عَـشَـرَ فَـقَـوْلُـهُـمْ يَـفْـسُـدُ بِـمَـا دَلَـلْـنَـا عَـلَـيْـهِ مِـنْ أَنَّ الْأَئِـمَّـةَ عَـلَـيْـهِـمُ الـسَّـلَامُ اثْـنَـا عَـشَـرَ فَـهَـذَا الْـقَـوْلُ يَـجِـبُ إِطِّـرَاحُـهُ عَـلَـى أَنَّ هَـذِهِ الْـفِـرَقَ كُـلَّـهَـا قَـدِ انْـقَـرَضَـتْ بِـحَـمْـد الـلَّـهِ وَلَـمْ يَـبْـقَ قَـائِـلٌ يَـقُـولُ بِـقَـوْلِـهَـا وَذَلِـكَ دَلِـيـلٌ عَـلَـى بُـطْـلَانِ هَـذِهِ الْأَقَـاوِيـلِ

“Now as for the one who says that the successor (Qa’im) has one descendant, and that the imams are thirteen, their statement is void by what we presented as an indication that the imams are twelve. Therefore the rejection of this statement is necessary. However, all these groups have died out, praise be to God, and no advocate of their statement has been left, and this is an indication of the falsity of these statements.” [Al-Ghaibah, page 142]

Comment:

Ash-Sharif-ul-Murtada thinks more imams are possible [Here!]