How does one bring to mind the intention for the night prayer (Salat al-Layl), and is it prayed two by two rak'ahs, and has a specific number of rak'ahs been mentioned for it, or is it open?

Intention (Niyyah) is the resolve, and its place is the heart. It is required at the beginning of every act of worship. For supererogatory prayers (Nawafil), specifying the particular prayer is not a condition; rather, a general intention for a voluntary prayer suffices. The supererogatory night prayer is performed two rak'ahs at a time.