3-5 is the norm for a "standard" day in my experience.
You can also make it 5-8 easy encounters if you want to have some attrition without extreme danger.
It tends to increase as you go up in level and everyone gets more spells, rage rounds, ki, judgements, etc.
Democritus is correct. It's worth bearing in mind that sometimes as GM, you'll have to "force" enough encounters on the party, otherwise they'll fall prey to the so-called "15-minute adventuring day". In wilderness environments this often means making sure that the players have to keep moving to reach a destination in a particular time-frame. In dungeons, wandering monsters are your friends.
Don't let the players decide the pace of encounters: making camp doesn't automatically make them safe. Save an encounter or two for when the party is resting. Just don't do it all the time (GMing is an art, in this respect - deciding when to throw encounters at the party is a judgement call).
Theoretically, the party should be able to handle 4 encounters per day equal to with CR=APL. This does not hold true at levels 1 or 2 in my experience or above level 12 or so. At 1 or 2 a lack of wealth, healing options, and duration of consumable abilities is usually too low to have that many encounters. Above level 12 you can often send extra encounters of CR=APL or increase the difficulty of each fight to higher CR.