You may have accidentally pressed 'cancel' in the light editor. Another theory is that your changes were not saved due to Em4 not being able to access and overwrite the file.
There is no limit to lights but having too many lights will cause lag when playing the vehicle. Many low to mid end computers cannot handle high poly models, or heavilly lighted models. It is recommended to stay under 90 lights (regardless of corona, radiation or polygon) in order to maintain a good FPS rate.