Is it better in supererogatory prayers to increase the number of rak'ahs or to prolong them, or is the reward calculated by time regardless of the number of rak'ahs?

If a specific characteristic regarding the manner of performing supererogatory prayers (Nawāfil) has been authentically established from the Prophet, peace and blessings be upon him, then it is sunnah to follow it. If a specific characteristic has not been established from him, then the scholars have differed as to whether it is better to prostrate more (كثرة السجود) or to prolong standing (طول القيام).

As for the Dhuha prayer, lightness (brevity) is part of its Sunnah.