(a) Out of Grace and Bounty, the Almighty Allah appoints prophets for man’s guidance. With regards to their number, Allah appoints them as per His Wisdom and Knowledge. We read in the holy Qur’an:
O children of Adam! If there comes to you prophets from among you relating to you My communications….. (Qur’an, 7:35)
Verily, appointment of an Imam for Imamate is a divine affair just as appointment of an Apostle is a divine affair. Even the Prophets have no right of selection. Likewise, their number is fixed by Almighty Allah as per His Wisdom and Grace.
(b) Whatever the number of the prophets or Imams, this question would still exist and you may again ask, ‘What is the logic behind this number? Why not more or less?
Thus, any affair whose very basis is in Allah’s Hand, its dimensions and other aspects are finalized by His Will and Wisdom; and we should surrender to His Will.