1999 ford explorer 5.0. How many miles does a engine last

Allot depends on how well it was maintained, how many owners?....was service done at scheduled intervals? do you have the service records? regularly serviced vehicles usually last longer than ones with a sketchy maintenance history... 217 k is about 11k a year not bad if you have service records, if so should go a while longer....just my opinion

If it runs good, has good compression and does not use oil it could run for a lot longer.

There's no guarantee, use high-mileage oil and keep a good eye on fluid levels, don't over-strain the motor or let it run hot, and just take it one day at a time. Be sure to change out old coolant as it loses its rust inhibitors over time. A rusted gunked up water jacket can shorten an old engines life. I've got a Crown vic (4.6L) with over 300K on it, and honestly still runs great. But i can also tell valve seats are going on it. It's not worth selling, it's BB value is nil at that mileage, so I'll just drive it into the ground and give it a Viking funeral when it finally does die.