Several questions on this site with accepted answers examine the experience, skill and particularly intent of wizards when casting spells against one another.
As such, while there aren't many concrete, canon examples beyond what @Gallifreian points out in his comment (Kingsley Shacklebolt using a non-verbal sheild charm to deflect/absorb two spells at once), I think a useful answer looks like this:
A very experienced wizard, such as Dumbledore (well-known for his superior power, knowledge and skill than most) should very easily be able to defend himself against a weaker, less experienced attacker - or more than one. Remember for example, his confidence against Ministry Aurors such as Dawlish. Remember also, Voldemort and Bellatrix were able to fend off three attackers each for a period of time during the final stages of the Battle of Hogwarts.
Similarly, a weaker, inexperienced wizard is unlikely to be able to defend himself against one more skilled attacker using a shield charm, let alone two or three. Picture the terror in most Hogwarts students who aim to match their good DADA OWL grades against a notorious Death Eater such as Bellatrix or Dolohov.
As in all interesting examples of combat, when similarly-matched duellers are facing off, the chances of success are far more difficult to call. If, say, Remus Lupin was duelling, say, Lucius Malfoy, I think the success and failure of spells is down to many fine factors as opposed to "Lupin uses Stupefy, which will definitely work, unless Malfoy deflects it at the right time with Protego." These factors are likely to include, but not be limited to:
-Innate magical power
-Levels of fatigue
-Intent - surely a killing curse from an enraged dark wizard is far more dangerous than one cast by a wizard who would rather not kill but feels they have to (remember Barty Crouch Jnr's comments on the attempts of a classroom of fourth-years who may attempt to using Avada Kedavra on him).
-Accuracy of casting - etc!
So, will a Shield Charm defend against more than one curse at a time? It depends on the power, experience and intent of the caster of both the offensive and defensive spell and various other factors during the moment of battle.