One at a time is advisable and it is more manageable than having multiple trades which makes you out of focus and priority.
Having several open trades is also not problem if the trader is able to understand good strategy, especially about TP and SL. These two option are mandatory for any trader who expects short-term returns, it is intended to prevent more losses when the market changes trend. So I don't think having 2-10 open trades is anything wrong.
The risk of losing lot of money is of course very high when the trader has several trades, but it will also be well worth the possible returns when the analysis is correct. But anyway, I don't like multiple trading even though I don't mind it being done by multiple traders.