There is no limitation on the number ordained to the office of Apostle. There is only the obvious limitation to the number comprising the Quorum of the Twelve Apostles. As a tangent, there is plenty of cases of note indicating that the counselors in the First Presidency need not be members of that Quorum or ordained Apostles at all.

In sustaining, “we sustain the counselors in the First Presidency and the Twelve Apostles as prophets, seers, and revelators”, with the President of the Church as “as prophet, seer, and revelator and President of The Church of Jesus Christ [...]“.

Now as is noted by the calling of members of the Quorums of Seventy to General and Area authority positions, along with sealing powers delegated under the direction of the President of the Church to those called as Temple Presidents, there’s a large degree of power and authority delegated to the Seventy as well.

But the Book of Mormon is vague at best as to whether they were ordained Apostles outside the set-apart Quorum of the Twelve Apostles at Jerusalem, or whether they were Seventy given power, authority, and special dispensations. And the old world is silent as to whether Peter and the Twelve were privately privy to the operations of the Church in the New World. It’s clear though that from the Book of Mormon account that general knowledge at least was retained from the body of the church at Jerusalem for reasons of unbelief and cultural narrow-mindedness.