Asr prayers are generally known as 15 rak'ats. Let's explain a little: 4 rak'ats sunnat: the first four rak'ats sunnat (sunnat zayeda) , which is rewarded if read, but not read is not a sin. 4 rak'ats are obligatory: then , the obligatory 4 (four) rak'ats which must be read. 2 rak'ats of sunnat: After 4 rak'ats of fard, 2 rak'ats of sunnat should be recited. The Prophet (peace and blessings of Allaah be upon him) never missed these two rak'ahs of Sunnah (Sunnah Mu'aqqada). 2 rak'at nafl: 2 rak'at nafl prayers are common in our society. In fact, naafil prayers can be performed at any time (except the forbidden time). 3 rakat bitir: then 3 rakat bitir prayers are read. It is not actually associated with Esha prayers. It is better to offer the last night prayers. However, many people fall asleep at this time because they cannot wake up on the last night. Note that the Biti prayer can be read in 3/5/6 rak'ahs.