Interestingly enough, I've only used 2 brokers ever. E-trade was my first account and then TDA.
I've been with TDA for about 5 years now.

Currently only 2 for redundancy, however I think it's interesting to be able to trade strategies and spreads over 2+ accounts (e.g., one for long and one for shorts). You just need to be careful to enter and exit effectively, and also having multiple accounts also impacts overall capitalisation. So I'd not recommend multiple accounts for beginners or any anyone without the needed capital to cover the potential margin requirements on each account.

Thanks,

Sandeep.

Two, one for Forex (Oanda) and the other one for futures and CFD's (IB). But the Forex one isn't really in use right know, due to the low volatility.