Are they all eggs with hard shells? I only ask because I have one of those 'freak' chickens who superovulates - some days she lays up to four eggs, but usually only the first of the day has a hard shell. The soft shelled ones cause her terrible problems - they get stuck, she strains terribly to get them out, and they sometimes break inside her.
So she's been really poorly actually, so it's more of a pathology than an interesting one-off laying event, so please don't worry about your odd extra egg. Our vet (who is an avian specialist) has never seen this level of superovulation in a chicken before in his entire career! We're looking into getting her a hysterectomy to put an end to her laying days once and for all.
So, to answer the original question: a hen can lay up to four eggs per day, in my experience... but she really shouldn't.