You can use ATM at two places in Everest Base Camp Trek. You can find them in Lukla and Namche. After Namche, there is no ATM booth. So, you are suggested to prepare the money you need during Everest Base Camp Trek in Kathmandu. But, at least, you can use the ATMs located in Lukla and Namche Bazaar in case of emergency and when you are running out of money on the trek.