There isn't one, single, objective "right answer" to this question: it depends on quite a number of variables. I think most people actively trading stocks are typically watching - even if only intermittently or once a day - a far higher number of instruments than most people actively trading other financial instruments. I trade and keep an eye on 5 different things at the moment, but more than 75% of my trades are taken on my preferred instrument (which is NQ - my others are CL, ES, and Euro and Pound futures, the last two of which I'm used to because I used to trade forex, mostly EUR/USD and GBP/USD, full-time). NQ alone would be "comfortable" for me, but at times when I can't trade that, I can find a few other opportunities elsewhere.