If you choose to buy tickets separately from a package (either from Disney or UndercoverTourist, etc.) just buy them for as many days as you need. If you decide to buy a package you'll just buy a longer ticket than your stay for as many days as you need (which is fine too).
For check-in day through check-out day you'll be able to book fastpasses 60 days in advance. For the other days at the end you'll book fastpasses 30 days in advance.
IF you stayed offsite first there would be an added step if you bought the tickets through a package but since you're staying on-site first I won't even get into all that.