from IRS. Link. https://www.irs.gov/retirement-plans/one-participant-401k-plans

Contribution limits in a one-participant 401(k) plan

The business owner wears two hats in a 401(k) plan: employee and employer. Contributions can be made to the plan in both capacities. The owner can contribute both:

• Elective deferrals up to 100% of compensation (“earned income” in the case of a self-employed individual) up to the annual contribution limit:
  • $23,000 in 2024 ($22,500 in 2023; $20,500 in 2022; $19,500 in 2020 and 2021), or $30,000 in 2023 ($27,000 in 2022; $26,000 in 2020 and 2021) if age 50 or over; plus
• Employer nonelective contributions up to:
  • 25% of compensation as defined by the plan