The dua of the Night of Decree
اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي
Allahumma innaka 'Afuwwun tuhibbu al-'afwa fa'fu 'anni
O Allah, You are Most Forgiving and love forgiveness, so forgive me.
Sunan al-Tirmidhi 3513 — sahih
Laylatul Qadr, the Night of Decree, is better than a thousand months. Worship on this one night outweighs more than eighty-three years of devotion.
Seek it in the last ten nights of Ramadan, and most intensely on the odd nights: the 21st, 23rd, 25th, 27th and 29th.
Because its exact night is hidden, the safest approach is to worship through all ten nights.
Pray qiyam, recite and reflect on the Qur'an, seek forgiveness, give charity and repeat the taught dua sincerely.
Aisha (may Allah be pleased with her) asked the Prophet ﷺ what to say if she found this night, and he taught her the dua below.
اللَّهُمَّ إِنَّكَ عَفُوٌّ تُحِبُّ الْعَفْوَ فَاعْفُ عَنِّي
Allahumma innaka 'Afuwwun tuhibbu al-'afwa fa'fu 'anni
O Allah, You are Most Forgiving and love forgiveness, so forgive me.
Sunan al-Tirmidhi 3513 — sahih
The 27th is emphasised in several narrations, but the night is deliberately concealed and can fall on any of the last ten nights.
Yes. Prayer and fasting are lifted, but dhikr, dua, istighfar, listening to the Qur'an and charity all remain, and the reward of this night is open to them.
There is no fixed number. Repeat it sincerely and often through the night alongside your own supplications.