Just going by the rough employee counts it would be 40%, 800k furloghed out of 2 million total. This whole business of forcing "essential" employees to work without pay, which would be illegal for any other employer to do, is keeping things from getting out of control for now.
Point of technicality, it's not "essential," it's "exempt from furlough."
There are plenty of government functions that are "essential" that have been mothballed for the shutdown. For example, there's no money for Federal Highway Grants, so the people who manage Federal Highway Grants are furloughed. When there's no funding, there's a lot fewer people needed.
The "exempt from furlough" employees constitute a much smaller pool of activities: those that can not be stopped. For example, FEMA employees that are responding to TS Karen, Border Patrol agents, TSA employees, VA Hospital workers.
