in the beginning, trading one symbol is great. if most people cant trade one stock successfully, how can they expect to trade twenty. the QQQ has plenty of volume, you can go direct via isld, no problem shorting(no uptick), etc. you can apply all the price/volume, pattern recognition methods and indicators that you want to one stock.. and another thing that is good for me as i approach 50 is that it doesnt move in hyper-speed; i know my limitations.... and just becuase you are only trading one symbol, doesnt mean you cant trade different accts in different time frames. and if fou have a swing or position trade, you're not gonna get whacked in 1/2 on a bad bit of news or SEC investigation. one symbol forces you to sit there and watch the price movement all day everyday, which might be the best way to learn.
but, if someone has been trading for 30 years, trading only the QQQs might not be the ticket...
BTW, if someone has decided to day-trade just one set-up to start in different stocks, then more power to you!