SP500 represents something like 85% of market capitalization. Adding a few more thousand stocks would make little difference, especially when you consider how heavily correlated they are. Just compare SPY with VT -- nearly the same thing.
That may not be what you want. Fortuantly computers can calculate the mix you want. mostly traders care about the stoks they follow they just want someeindication if the unexpected movement they are seeing is the market or something they don't know.
As I understand it, they don't own every stock. Right now VTI has about 3700 stocks out of the ~4300 on US publicly traded markets (excluding OTC). But it's more representative than the S&P 500 and similar.
