Is not about how many trades, is about how much are you risking in total.
Let me give you an example. Lets say that your account is 500.00USD and you open a SELL position on GBPJPY, and lets pretend that your lot size is 0.01 and your stop loss is 50 pips and your take profit is 250 pips.
If your trade touches your stop loss, then you would have loss 5 dollars, which represent 5% of your account. If you would have opened more positions and they all touch stop loss, you would have loss a higher percentage of the account.
Ideally, you should risk 1% or less of your account and the maximum should be 3%.
In conclusion, once you opened several trades, check how much would you lose if they all touch stop loss if the result is more than 3%, you had opened way to many trades.
Personally, I only have one trade open at a time.