The Railpass (Interrail Eurail is your ticket) you can buy the mobile pass at anytime even just minutes before the start (You still need time to regist your Railpass with your ID-Card/Passport)
Reservations (that sometimes must be paid on top of the Railpass) depends from Country to Country :) Paperreservations via Interrail atleast 9days in advance E-Reservations via Interrail atleast 3hours in advance Reservatns at stations or online at the operator website often till last minute :)
Some Trains just have limited seats for Railpassuser like Eurostar to from London, TGV from France to Spain or Thalys between Paris - Brussels/Amsterdam/Cologne and are often sold out weeks in advance in high season like summer or christmas :/