Myth
Congress will decide how votes are cast and counted. It will probably look like the Electoral College.
Fact
Neither Congress's power to "call" the convention, nor its Necessary & Proper powers can be used to prescribe how votes will be counted. All states will be given equal voting strength because a convention of states is a meeting of equal sovereigns.
Call does not mean control
A review of founding era convention calls reveals that the power to call a convention is limited to specifying the time, place and purpose for the convention (source).
Founding-Era calls did not try to control the composition, rules, or conduct of the convention beyond designating time, place, and purpose (source).
Congress may not use its Necessary & Proper powers to control the convention.
The Necessary & Proper clause of the Constitution only applies to the powers enumerated in Article I, Section 8 (source). With respect to the amendment process, statutory law is advisory only (Dyer v Blair)
In another case (Idaho v Freeman), the courts held that
Congress' power to participate in the amendment process stems solely from article V…
Congress, outside of the authority granted by article V, has no power to act with regard to an amendment, i.e., it does not retain any of its traditional authority vested in it by article Isource: https://www.leagle.com/decision/19811636529fsupp110711473
Why equal voting?
Because a convention of states is a meeting of equal sovereigns. States at a convention would each have the same voting strength similar to the way member countries in NATO or the UN have equal voting strength.
It's always been that way
Every interstate convention that's ever been held in America was one state/one vote. The only exception being a convention in the late 1800's where each state got 8 votes. But even in that case, no state had a greater voting strength than any other state.
Practical reasons
There are also practical reasons why states would all have the same voting strength. The notion of giving some states greater voting strength than others was actually considered in the 1780's. For an more in-depth explanation of why that failed, read this article.
The Founders' intent
We also know what the founders intended from the ratification debates : one state, one vote and majority rules at a convention. From the debates in Massachusetts, we read...
...seven States, assembled in Convention, as proposed, agree to any amendments...
