An extract from Usool al-Deen al-Islami by Shaykh Muhammad ibn Ibraaheem al-Tuwayjri posted at islamqa states:
When he saw how stubborn they were and how the signs of kufr were manifest among them, he stood among his people, saying, Who will be my helpers in the cause of Allaah? The disciples believed in him, and their number was twelve.
And the following ayahs are cited:
“Then when ‘Eesa (Jesus) came to know of their disbelief, he said: ‘Who will be my helpers in Allaah’s Cause?’ Al-Hawaariyyoon (the disciples) said: ‘We are the helpers of Allaah; we believe in Allaah, and bear witness that we are Muslims (i.e. we submit to Allaah).’
Our Lord! We believe in what You have sent down, and we follow the Messenger [‘Eesa (Jesus)]; so write us down among those who bear witness (to the truth, i.e. Laa ilaaha illallaah — none has the right to be worshipped but Allaah)”[Aal ‘Imraan 3:52-53]
However, none of the ayahs speaks about the total number of the disciples. Moreover, the number might have been mentioned by the Prophet but it's not cited in the excerpt nor did I ever came across that hadeeth.
Does anyone know the evidence where that number is derived from?
Oddly enough, the Bible also mentions twelve disciples.