What is the ruling regarding my prayer in which I doubted the number of Rak'ahs and favored the larger number, but did not perform the prostration of forgetfulness (sujud al-sahw)? And what should I do if I don't know exactly which prayer it was, though I predominantly believe it was the Maghrib prayer?

If you built upon the greater number after it became preponderant in your estimation, then your prayer is valid. This is the legal ruling according to a group of scholars, as stated in "Al-Sharh Al-Mumti'" by Sheikh Ibn Uthaymeen. So, if you doubt the number of rak'ahs and one of the possibilities becomes preponderant in your estimation, act upon it and perform two prostrations (of forgetfulness) after the salutation. However, abandoning the prostrations of forgetfulness after the salutation does not invalidate the prayer.