It is very important to book this trek a good time in advance!!! As there are only 500 permits (Tour Guides, Porters and Cooks including) issued per day for the Inca Trail
A general recommendation on how long time in advance to book is:
All companies managing the Inca Trail must be certified and have an Inca Trail operators license. This license is issued annually each October - November. There are currently 185 registered operators (2015), with new agencies springing up each year. Many of these companies, however, may only operate a trek a couple of times a month during the high season and these treks are mainly private groups for overseas tour companies. Before you hurriedly sign up with any agency, its vital to comprehend some things about the industry. The most important thing is, that not all agencies are licensed tour operators.