D&C 107

It is in this revelation, that the Lord provides for the growth of the Church in the latter days.  This becomes apparent as the Lord outlines the calling and duty of the Seventy -

"And it is according to the vision showing the order of the Seventy, that they should have seven presidents to preside over them, chosen out of the number of the seventy;
94 And the seventh president of these presidents is to preside over the six;
95 And these seven presidents are to choose other seventy besides the first seventy to whom they belong, and are to preside over them;
96 And also other seventy, until seven times seventy, if the labor in the vineyard of necessity requires it.
97 And these seventy are to be traveling ministers, unto the Gentiles first and also unto the Jews."  (Doctrine and Covenants | Section 107:93 - 97)

It appears that when the Lord references the office of Seventy n the Melchizedek priesthood, He uses the term "Seventy" (with a capital letter).  When He is speaking in reference to quantity, He uses the term "seventy".

It is in this revelation that the seven presidents of the Seventy are to preside over all quorums of seventy.  And that the number of quorums of seventy would be in relationship to the needs of the growth.  And in addition, the Lord also allowed -

"Whereas other officers of the church, who belong not unto the Twelve, neither to the Seventy, are not under the responsibility to travel among all nations, but are to travel as their circumstances shall allow, notwithstanding they may hold as high and responsible offices in the church."  (Doctrine and Covenants | Section 107:98)

We have seen this in the past with the callings of Assistants to the Quorum of the Twelve, and regional representatives.