The permit process requires confirmation of adequate health insurance covering three specific things: emergency evacuation for a condition or injury that can't be treated in the territory, transport and medical care in transit to the nearest country with suitable treatment, and repatriation of mortal remains if a visitor dies. The government does not publish one universal numerical limit for this on its permit page.
Because there's no published figure, the practical route is to ask your cruise operator for its required amount, then have a licensed insurance professional confirm the policy meets the wording of the South Georgia requirement specifically, not just a general evacuation limit.
The context behind the wording is worth knowing. South Georgia has no visitor medical facilities and no independent search and rescue service. Evacuation can take days.