in a room of 10m x 20m and height 5m high, if we get an airflow of 1m / s through an inlet of 1mx1m, what setting to change so that it has several lines rather than a single line between inlet and output? (using Rans flow)
I presume by "several lines" you mean streamlines. I would suggest applying the Conditions->Accuracy tool on the Physics Tool Palette to your inlet to increase the number of mesh points. For more details see "Accuracy Tool."
If this doesn't help then gradually decrease the Particles->Increment property. For more details see "Particles or Streamlines."
I'd also suggest you try the basic tutorial "Incompressible Flow Through a Pipe into a Box" which creates some streamlines.