The plural of index is indexes or indices.
1. A list, catalogue or directory ordered alphabetically or numerically. Examples include lists of citations at the end of an article or book and lists of publicly traded stocks.
2. In mathematics, an index number can refer to a number or expression which is raised to a power. Index numbers are also known as exponents.
The plural for index can be formed in two different ways depending on the definition of the word in context.
When used to refer to multiple lists or directories, the word indexes is used. This follows the regular English rule for pluralization by adding the suffix -es to the end of the word.
When used is a mathematical sense, the irregular plural of indices is preferred.
The rules however, are not completely clear-cut and the words can be used interchangeably, especially when used informally.
American informal English tends to be prefer the word indexes whilst British English uses indices.
In formal, academic and financial contexts, both prefer the use of the word indices.
Here are some examples to illustrate the difference between the singular form "index", the plural form "indexes", and the alternative plural form "indices":
The graph shows the occurances of the plural of index in written English since 1800 using Google's Ngram Viewer.