The benefit of trading more than one lot is that you can cut down your position once certain profit targets are hit. That way you can create risk free trades and possibly capture more profits. You lot size should really depend on your risk tolerance though. If you have $300 in a forex account trading 200:1 margin. You should definitely only trade one lot at most. In my opinion you shouldn't even be trading unless you have at least 5k for a mini lot... save up and learn to trade right first before you use real money