The Alter Rebbe usually said two Maamorim (presumably one on each night) each year. However, there were occasions when he said three or four, and we actually have five Maamorim which were all said in the year 5564 (1804).
The Mitteler Rebbe, who was known for his lengthy and very explanatory Discourses, would often say several Maamorim even on a regular Shabbos. One Shavous he said a total of eleven Maamorim, each of them very lengthy. His uncle Reh Yehudah Leib (the Alter Rebbe’s brother, known as the Maharil) gently chided him, saying, “Nit aleh hobin dein kop!” [“Not everyone has a head like yours!”] (Beis Rebbi, Vol. 2 pp 3a.)
One year after Shacharis the Mitteler Rebbe began to say a Maamer, explaining the concepts at great length as was his custom. When he finished the Maamer, the Chassidim began to disperse but quickly relised that the Rebbe was about to begin a second Maamer. Again, the Rebbe spoke at great length and when he finished it was apparent that he was preparing himself to begin a third. Seeing this, Reb Hillel Paritcher ran to the house of the Tzemach Tzedek [who did not always attend, his father-in-law – the Mitteler Rebbe’s, Maamorim], unable to restrain himself he grabbed the Tzemach Tzedek by the beard and cried, “Mendel Mendel! Kum her on zeh vi es gist zich chassidus in gass!” [“Mendel Mendel! Come, hear and see how Chassidus is pouring out in the street!”] (Migdal Oiz, pp 187)
The Tzemach Tzedek
In the year 5607 (1847) many Chassidim came to spend Shavuos [with the Tzemach Tzedek] amongst them the most famous, Reb Hillel Paritcher and Reb Betzalel Mai’azaritz, as well as many others. After coming from the Mikveh on Eruv Shavuos, the Rebbe said a Maamer beginning “Anoichi Havyeh Eloikechoh” [“I am Heshem your G-d” the first of the Ten Commandments]. The son’s of the Rebbe spent the entire evening and night, each in his own house, reviewing the Maamer [before the guests] and before dawn Reb Bezalel repeated the entire Maamer from memory.
After Davening there was a festive meal and all the great Chassidim and Rabbonim, as well as the sons of the Rebbe, were invited… those who were not invited stood…. When all the invitees were seated the Rebbe came out of his room and sat at the table… he poured some wine into his cup, made a brocho and said L’Chaim to his sons and the other invitees, nodding his holy head to all those who stood. He began to sing and his sons – not the invitees – sang with him. It was extremely gratifying to hear their melody. Then the Rebbe rose from his chair and began to dance alone where he stood, he held a handkerchief in his hand [it was the custom of the Rabbeim to wrap their hand in a handkerchief while sayin a Maamer]. The sons of the Rebbe – but not the invitees – clapped their hands in accompaniment…