Daytime: two or four per salam; more than four is disliked.
Nighttime according to Abu Hanifa: up to eight per salam is permissible, and more is disliked.
Nighttime according to Abu Yusuf and Muhammad: two rakahs per salam.
The disagreement concerns how rakahs are joined under a single salam, not the absolute number of voluntary rakahs permitted over the night.
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Voluntary Prayers: Regular Sunnahs, Nawafil and Juristic Differences
Daytime: two or four per salam; more than four is disliked.
Nighttime according to Abu Hanifa: up to eight per salam is permissible, and more is disliked.
Nighttime according to Abu Yusuf and Muhammad: two rakahs per salam.
The disagreement concerns how rakahs are joined under a single salam, not the absolute number of voluntary rakahs permitted over the night.