The actual UPS configuration required is chosen by run time and energy usage, and energy usage is dependant on the configuration of drives and shelves.
If you have a FAS2554 with 24 x 6TB internal drives, worst case power usage at 110V is 650W, with 560W being typical.
For a typical 15 minute minimum run time, supporting only this single shelf configuration, you would be looking for a 1500VA/1200W UPS that might only be rated for 5 minutes at full load, but 12 minutes or more at half load.