It is a well known fact that Sri Ramanujar established 74 simhAsanAdhipatis to propagate the Srivaishnava Visishtadvaitha Sampradhayam. It is no coincidence that he selected this number - in fact, there is a specific reason behind the selection.
Sri Nathamuni when he was learning the 4000 verses of Divyaprabandham from Swami Nammazhvar, was also informed of the coming of Swami Ramanujar by Azhvar. Sri Nammazhvar also told him to instruct and prepare Sri Ramanujar through his grandson Sri Alavandar. Occasionally during his life Sri Nathamuni would refer to this by stating "Gathered under a hoof, sparrows would drink; gathered in the Veeranam Lake, the country would prosper". The meaning is that, rain water pooling in the ground under the hoof of an animal would be drunk by just a few sparrows. The same rain water when falling into the Veera Narayanapuram Lake (Veeranam Lake) would then be used to irrigate large tracts of land and therefore be useful to the whole country. Similarly, until the time of Sri Ramanujar all acharyas would benefit only a few select disciples; from Sri Ramanujar the true meanings of Srivaishnavam would reach the world.
That Sri Nathamuni would say this was related through his disciples to Sri Alavandar and then through his disciples to Sri Ramanujar. Noting that Sri Nathamuni thought of him as the Veeranam Lake with its 74 outlets of water, Sri Ramanuja decided to spread the Srivaishnava message through 74 simhAsanAdhipatis.
It is also of great interest to note that Sri Manavala Mamunigal's Upadesarattinamalai, which brings meanings of Srivaishnavam to us while teaching us about Azhvars and Acharyas, is also made up of 74 verses.