Can you only buy one extra die with the The Calm Before The Storm destiny square?
You can only buy a single die, per Threat Phase.
The Destiny Board says:

Any Die Score: Roll an extra Die of Destiny during the next Threat Phase.

Rules page 8

(red) The Calm Before The Storm (red) allows the Demon player to roll an extra Die of Destiny during their next Threat phase.
Abilities printed in (red) can only be selected once per game Threat Phase (Errata: claustrophobia-theboardgame.com/fr/?p=441). There are no limitations for the Abilities printed in (black).