Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
at its height, the inca empire which controlled some twelve million people was the largest empire in the world.

Explanation:

1. Identify the relative clause: which controlled some twelve million people.
2. Determine if it is restrictive or nonrestrictive: The clause provides extra, non-essential information about the Inca Empire (the empire is already uniquely identified, so the clause does not define or narrow it down). This makes it a nonrestrictive relative clause, which requires commas to set it off from the main sentence.

Answer:

At its height, the Inca Empire, which controlled some twelve million people, was the largest empire in the world.