It is not accurate to assume you're having multiples based on your HCG levels. A sonogram is the way to go!
amandacole posted a great chart! Also, if you google betabase you can find "average levels" that women have reported for single, twin, and triplet pregnancies.
Try not to read too much into HCG levels though. Although its fun to compare the only way to tell for sure how many babies are there is u/s.
hCG levels in weeks from LMP (gestational age)* :
3 weeks LMP: 5 - 50 mIU/ml
4 weeks LMP: 5 - 426 mIU/ml
5 weeks LMP: 18 - 7,340 mIU/ml
6 weeks LMP: 1,080 - 56,500 mIU/ml
7 - 8 weeks LMP: 7,650 - 229,000 mIU/ml
9 - 12 weeks LMP: 25,700 - 288,000 mIU/ml
13 - 16 weeks LMP: 13,300 - 254,000 mIU/ml
17 - 24 weeks LMP: 4,060 - 165,400 mIU/ml
25 - 40 weeks LMP: 3,640 - 117,000 mIU/ml
Non-pregnant females: <5.0 mIU/ml
Postmenopausal: <9.5 mIU/ml
really thats interesting to know cause i wasnt sure what was normal
that is really high! at 5 weeks my beta was 3570