EDIT: that's not right. please see below.
Individuals can buy $10K/yr. Married couples can each buy $10K/yr, and you can buy the same amount for children.
Also, businesses can buy $10K/yr, trusts can buy $10K/yr, and you can purchase $5K/yr with a tax refund.
It would be not unusual to be able to buy $45K/yr (4-person household), and if one of the family is self-employed, $55K/yr.
....
That said, don't buy lots of I bonds without first considering the advantages of TIPS!
https://www.treasurydirect.gov/indiv/research/indepth/ibonds...
EDIT: that's not right. please see below.