The emergency fund is the first layer of personal financial safety. The usual rule: three to six months of basic expenses, kept in a liquid and accessible account, separate from day-to-day spending.
Its job is not to earn returns; it is to be available when you need it: car breakdown, sudden job loss, medical bill. If you invest it in something volatile, it stops doing its job.
When is it “enough”: when it covers your fixed expenses for three full months without touching your salary. Beyond that, additional savings can start being invested.