1)In webrtc at initial state we pass sdp protocol between peers to establish a connection.Does it happen only once or it happens through out the call process?
2)How does it calculate the peers bandwidth?Can we change it dynamically?
SDP for video is passed only once, exactly the same way as for an audio-only call. You can suggest bandwidth with the SDP and depending on the codec it might also change at run-time, there is no need for extra SDP exchange for this.
SDP can be exchanged also multiple times during a session, but this is only for special functionality if you need to change the media parameters such as re-invite with different codec, hold/reload or conference.