Add more dates where? in between the values? After the last date?
First up, your chart has 13 steps on the X axis - the first being Sep 25, and the last 8 being blank. That implies your source table has 13 rows, but only the first 5 have data points.
So the first answer to your question is that the chart will give you steps based on the source data - thirteen rows => thirteen steps on the axis. You just need to have values in them.
Secondly, and slightly off topic, but you likely shouldn't use a line chart for dates. That's because each point on the chart represents a row from the chart, but the intervals are not consistent - for example, there are 44 days between between points 1 and 2, only two days between points 2 and 3, followed by 1-day increments. This could be misleading if you're looking for trends over time.
Instead consider using a scattergraph. For a scattergraph, Numbers will interpret dates correctly and scale the axis accordingly:

This shows a better relationship between the first and second data points, spread over a longer timespan than the other data points.
Additionally, with Scattergraphs, you can set the Min and Max dates for the chart and the number of steps via the sidebar:

The line chart, by contrast, is based on the number of data points in the source data.