EDIT: Follow up question: how do the numbers compare to the deaths of other species during the Battle of Korriban, and other comparable espionage missions?
I've always found the official numbers to be highly suspect due to the entrenched prejudices of the Rebel Alliance's political and military leadership.
"Bothan spies." Nobody says "human pilots."
No Bothans died to bring the information to the Rebel Alliance. The operation was a sham to lead the Rebel Alliance to gather their forces into a trap.
Legends sources estimate the number to be around two dozen.