It is recommended that at least 50-75 candles be showing on the chart to obtain a better overview of how the pair has been moving over time.
When a trader "expands" a chart and just looks at 15 to 20 candles or less, they are doing themselves a disservice in that they cannot see the big picture and may not be able to see prior support/resistance levels, for example, that could very well impact their trade.
Take a look at the chart examples below...
The "expanded" chart includes about 60 candles. Even so, you can see how the full view of the chart provides much better insight for the trader. In this case, trendline support which clearly plays a part in the analysis of this USDCHF