Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

How much of each stock do index funds hold?
Note that an index fund may not be able to precisely mirror the index it's tracking. If enough many people invest enough money into funds based on that index, there may not always be sufficient shares available of every stock included in the index for the fund to both accept additional investment and track the index precisely.  This is one of the places where the details of one index fund may differ from another even when they're following the same index. IDEALLY they ought to deliver the same returns, but in practical terms they're going to diverge a bit. (Personally, as long as I'm getting "market rate of return" or better on average across all my funds, at a risk I'm comfortable with, I honestly don't care enough to try to optimize it further. Pick a distribution based on some stochastic modelling tools, rebalance periodically to maintain that distribution, and otherwise ignore it. That's very much to the taste of someone like me who wants the savings to work for him rather than vice versa.)