A 1099-INT form reports interest received during the year. You will receive a 1099-INT if your interest amounts to $10 or more over the course of the year. If the amount of dividends received is below $10, a 1099-INT is required if there were any federal taxes paid due to backup withholding.
Please note that while Betterment is generally not required to issue you a 1099-INT if you earned less than $10 of interest from your account, the IRS requires taxpayers are still required by the IRS to report and pay taxes on any earned income. You can see your interest history from the Activity page in your account here.
Betterment is not a tax advisor, nor should any information herein be considered tax advice. Please consult a qualified tax professional.