No, you're not obligated to raise a certain number of funds from sponsors. You're certainly encouraged to find sponsors on your own who are willing to support and motivate you during NaNo, but there are no minimum requirements for participation. I'm usually able to ensure everyone has at least one sponsor by the time everybody starts writing... but the more sponsors you can find on your own, the better the likelihood of winning or earning one of the MBs offered in the Prizes section.