It's never a good idea to have all your eggs in a single basket. That's good advice in any field, not just trading. So in general, when should you start spreading out your net worth among several brokers? When you reach a six-figure account? Seven?
I use two brokers. TradeStation is my main trading platform but I use E*Trade as a back up in case of disconnection during a day-trade. It also reduces my commission fees. TradeStation on a per share basis, so good for less than 1200 shares but E*Trade fixed commission of $9.99 per trade is cheaper when I trade more shares at once, i.e. above 1,200 shares per trade.
I guess it would depend what you are doing with the money. I have accounts at three different firms, but it's because each has certain qualities that lend themselves to particular strategies. One has a very good api, allows me to trade multiple instruments and has a good short inventory. One offers me a lot of leverage. One gives me cheap carrying costs on long term positions. When I want to trade particular strategies, I select the broker that will best accomodate them.