How much should an emergency fund be?
A common guideline is 3 to 6 months of essential expenses — rent or mortgage, utilities, groceries, insurance, transport and minimum debt payments. Aim nearer 6 months (or more) if your income is irregular, you're the sole earner, or your job would be slow to replace.
Where to keep it
Keep it somewhere safe and instantly accessible, like a high-yield savings account — not invested in stocks, where a crash could coincide with the emergency.