A permit system restricts the number of trekker taking the Classic Inca trail to 250 per day but this number includes all the guides and crew members who also need a permit. As a result, the number of permits available for trekkers is only about 200 each day.
For reasons nobody quite understand, the Park Authority issues all the permits for each year available on one fixed date. For 2019, all the permits are to be put on sale in October. You cannot book a permit for a 2019 trek until that date and within the days of the permit sale, they will be sold out for the high season.
The critical thing to understand is that prior to that date, no operator can guarantee that you will get a permit. If an operator informs you that they are able to book or pre-order permits, they are lying. The permit system is policed very strictly and permits are only issued against specific names and passport numbers. You are required to check-in for the trail at the 82 km starting point with your passport. If you got married recently, you must take a passport in your maiden name as the park authorities are not at all flexible.