How much cash can the average ATM hold?

The average retail ATM that you would see at a convenience store, gas station, or hotel would typically be stocked with a maximum of $20,000. Most ATM operators use a machine with a cash cassette with a 1000 note capacity and they typically load the ATM with $20 bills.
$20 x 1000 notes = $20,000 maximum.

Very busy retail ATM locations (at convenience stores, shops, etc..) may have ATMs with larger cassettes. Some retail ATMs can hold up to 8000 notes. They have four cassettes that hold up to 2000 notes each. While it would be extremely uncommon, it is possible that the ATM could dispense only $100 bills (instead of the usual $20 bills) in which case the retail ATM could hold up to $800,000.
8000 notes x $100 = $800,000