It is Sunnah to make these Nafl prayers brief: 1. Two Rak`ahs before Fajr prayer 2. Tahiyyat Al-masjid 3. Starting voluntary night prayers with two brief Rak`ahs 4. Two Rak`ahs following Tawaf. It is Sunnah to make the following prayers lengthy: 1. The eclipse prayer 2. Qiyam Al-layl With regard to Sunnah prayers other than that, one should do what helps him be more focused.

If you missed Fajr prayer, but made it up after sunrise, then you may pray Duha because Duha prayer is a confirmed Sunnah that has no connection to Fajr prayer whether it was done on time or made up later.

The minimum number that has been reported in the case of Duha prayer is two rak‘ahs. There is no text that defines the maximum number of rak’ahs for Duha prayer, but it is proven that the Prophet prayed Duha with four rak‘ahs, and with more than that, and with eight rak‘ahs, as on the day of the conquest of Makkah. For more about the number of rak`ahs in Duha prayer, see the detailed answer.