This depends on individuals. With $10M investable asset, I would open accounts with no more 3 major brokerage houses, for example, Fidelity, Vanguard and Charles Schwab. Evan though, you have accounts with these 3 firms, it does not mean that all your money is with them. The reason being is that you buy mutual funds and the money is actually with the mutual fund companies, not with your brokerage companies.
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