When the party has to travel over an expanse of land, what should you do? In theory, most player would want to get to their destination and continue with the main story arc, but you also don't want it to feel like the party can fast travel across a map without an consequence or tribulation. I feel that most DM's would probably add the obligatory one encounter, but what sort of thing do you add? Do you add an irrelevant fight versus an animal? Do you add a potential side-quest (probably most fitting if you're only doing the single-encounter approach)? Do you place the characters in a situation where they need to decide what side to be on (not necessarily Hatfields and McCoys, but assist a fleeing prisoner or turn him into the pursuing guards)?
Regardless of the options, you really have to base it on your party's Modus Operandi (M.O.). If your party tends to be a hack-n-slash group, it might be best to perhaps throw a possible dungeon crawl at them. A party that tends to be on the roleplaying side would probably fare better with a decision tree encounter where how they respond will affect the outcome of the encounter. Of course there are all sorts of party distributions including treasure-hunters, power-mongers, riddle-masters, questers, etc. that would prefer different sorts of encounters, but it really comes down to how engaged the party is into how many encounters you should have.
My large group is traveling from Suzail to Baldur's Gate (Forgotten Realms) and will probably have about a 2 week journey out there. The party configuration is a mix between questers and hack-n-slash so, though I don't have the details yet, I think the best choice of an encounter could potentially be a protection mission to get someone or something from point A to point B without harm. This way the quester portion will want to succeed in the objective and in theory the hack-n-slash portion will be more than willing to protect by whatever means necessary.
My small group was traveling across the SE seas of the Forgotten Realms and they are closer to a role-playing group, but also have rich backstories to use to my advantage. This time during their travels they were stopped and attacked by an old acquaintance of one of the party members who was determined to duel and who wanted revenge and would search the seas until his revenge could be exacted. Ultimately the duel was not finished as one of the ships began sinking, but this will probably be the only encounter for that travel and it filled out their journey more than just fast traveling.
Ultimately, you as the DM have the say to see how many encounters you want to throw at the party. Perhaps you can have some encounters prepared and if it doesn't seem like your party is engaged in them then you can always save those for another time. No one was ever sorry for being over-prepared. I acknowledge that I really never gave a number of what a good amount of encounters would be, but really I can't without knowing what the M.O. of the party is. Hopefully when you plan out your encounters you can think of good options to flesh it out so that the players are engrossed in the action and just aren't rolling their way through an obligatory encounter.