Maghrib prayer has 3 rak'ats or rakats in total. These tree rakats are obligatory. In the first 2 two rak'ats, one should recite Surah Ikhlas or any other Surah, and 1 one must be left blank. There are 2 two rakats in it as Sunnah, if you recite them, you will be rewarded. If they do not fall, there is no sin.