I only watch /ES and /CL, with 80% of my attention being favored towards /ES. I tried watching bonds, currencies, gold, etc. and it was just too much for me. I chose /ES because of how liquid it is and /CL for the volatility moves.
Currently, I'm trading 3 instruments -- a cointegrated triplet of ETFs. I look for a divergence from equilibrium of this portfolio and bet on it's return to equilibrium. It's similar to a Bollinger Band approach or pair trading.
I trade 4, most trades happen on crude oil and nasdaq.
Other 2 instruments i watch are S&P and natural gas, but i trade these not as much as the instruments mentioned above.\
I do use a multiple screen setup, if i would only have 2 screens i would only watch 2 instruments.
I find it very disturbing and not good for my concentration to have to click trough different charts.