>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.