From the IRS:
https://www.irs.gov/retirement-plans/one-participant-401k-pl..."""
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:
$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, or
for self-employed individuals, see discussion below
If you’ve exceeded the limit for elective deferrals in your 401(k) plan, find out how to correct this mistake.
Total contributions to a participant’s account, not counting catch-up contributions for those age 50 and over, cannot exceed $66,000 for 2023 ($61,000 for 2022; $58,000 for 2021; $57,000 for 2020).
Example: Ben, age 51, earned $50,000 in W-2 wages from his S Corporation in 2020. He deferred $19,500 in regular elective deferrals plus $6,500 in catch-up contributions to the 401(k) plan. His business contributed 25% of his compensation to the plan, $12,500. Total contributions to the plan for 2020 were $38,500. This is the maximum that can be contributed to the plan for Ben for 2019.
A business owner who is also employed by a second company and participating in its 401(k) plan should bear in mind that his limits on elective deferrals are by person, not by plan. He must consider the limit for all elective deferrals he makes during a year.
"""
Per company means that if you contract for one company (with your own LLC) and work W2 for another (or work 2 W2s), you get ~22k total for employee contributions, but ~40k per each company you work for, ie 2 * 40k = 80k, therefore you can make 80k + 22k = 102k entirely into your 401k accounts. However, since it's a max of 25% per plan of each income, you'd have to be making very high six figures for it to get to such a maximum.
If you have your own LLC, you can set up a solo 401k easily, but for an employer, it's much harder as they must support mega backdoor Roth IRAs, ie allowing you to add your own after-tax money as part of their employer contribution.