Unfortunately, hCG numbers aren't too helpful for judging the likelihood of multiples. The actual levels can vary a lot (I knew a woman with twins whose first hCG reading, right around two weeks after conception or 4 weeks' gestation, was only 70.)
I'd pay more attention to the speed of doubling rather than the actual level. Both times I was pregnant with twins, my hCG increased more by more than double in 48 hours. However, this also happened to a girlfriend of mine who was pregnant with just one healthy and hearty bub.
Hate to say it, but embies can start and then fade, and embies can split also, so don't get yourself worked up until you see by ultrasound what is going on. Some women have high numbers and fast-rising levels and still have just a singleton.
Oh and I'm saying 4 weeks because they add 2 weeks to pregnancy from when you actually concieved. So even if the transfer was only 12 days ago you'd be at almost 4 weeks.
I'm not sure how far along you would be? 4 weeks? At 5w3d my levels were 11000 and at 5w6d they were 32000. The range for "normal" in pregnancy can differ greatly. Its possible that you have more then one in there or it's also possible there is just one. Only an ultrasound will determine.