I was wandering if I can change the number of cars in veins or is fixed? If so, which class in the code I can find the declaration of the number of cars?
If you use randomTrips.py to generate rou.xml you can specify the number of vehicles you want in your simulation. In this case a 50 vehicles rou.xml file will be created.
<SUMO_HOME>/tools/randomTrips.py -n input_net.net.xml -e 50