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Actually, $15,000/year -- you can buy an additional $5,000 in paper I-bonds with your federal tax refund, and you can arbitrarily overfund your tax payment to be sure to have $5,000 in refund. The paper bonds can be exchanged for electronic I-bonds through a somewhat annoying process.

Do you know what % of that is fixed vs. the inflation adjusted rate? Also, I was confused by how often the inflation rate adjusts the interest rate.

The fixed rate is 0.00% -- it's all inflation rate. Inflation rate is adjusted every six months.

Here are past fixed rates going back to 1998: https://www.treasurydirect.gov/indiv/research/indepth/ibonds... It hasn't been much above 0% since 2008.

Do you really need to buy these bonds with payroll deduction?

No.