Contributions to your 401(k) or TSP plan will show up in box 12 of your W-2 form, with the letter code D.
Annual employee contribution limit (2022): $20,500
Additional catch-up contributions for those age 50 and older (2022): $6,500
Income limits for contributions: None, but employee contributions can not exceed wages
Matching contributions: Depends on your employer's plan
The deadline for electing to make an employee contribution to an individual 401(k) is December 31 and the employee contribution should be made as soon as is practical after that. The deadline for making an employer contribution is the due date of your tax return, including extensions.