>There are 24 upavyūhas of Viṣṇu in the Pāñcarātra tradition,
>distinguished by the order in which their four hands display the discus,
>conch, club and lotus flower. (I regret that I have no textual source
>for this -- perhaps someone else does?)

--------Martin Gansten
 
http://creative.sulekha.com/vishnu-dwadashanamas-part-four_426742_blog
 
mentions
 
 Padma Samhita (Jnana kanda: 2, 26- 28)

as the source for Chaturvimshati murti.
 
The blogger gives a very detailed analysis of the topic.
 
interestingly there is a table connecting the 24 upavyuhas to 24 letters of the Gayatri Mantra.
--
Prof.Nagaraj Paturi