The Inca trail is a protected area of Peru, and you are required by law to have a trekking permit. Permits are purchased by your trekking company and are usually part of price you pay.
Permits are limited to 500 per day and this includes hikers, guides and porters. Typically there are 1.5 porters for every paying trekker, therefore the actual number of permits available for trekkers is only a few hundred. Booking early in high season is important to avoid disappointment. Planning 3 months in advance is not unusual.
The DRC website is the online booking system that Inca Trail trekking companies use to book permits. You can enter the site as a guest to view availability. Be careful as this website is not updated every day, and the number of displayed permits can sometimes be inaccurate. Click here to visit the website: http://www.machupicchu.gob.pe/