No more than 2% of your account balance on any one open position.
Risk is measured as the distance between your entry price and your stop-loss, multiplied by your volume.
On a $100,000 account, 2% is $2,000. If your stop-loss is 100 pips away on EURUSD, one standard lot puts $1,000 at risk — that is 1%. Two lots would be $2,000, which is exactly at the limit.