In year 2019, you can defer the LESSER of 100% of your compensation or $19,000. And if you will be age 50 or over by December 31, you can contribute an additional “catch-up” of $,6000 for a total of $25,000.
If your 401(k) plan allows you to make Roth contributions, the total of your Roth and regular contributions cannot exceed these dollar limits.
Your Employer will make sure you do not exceed the appropriate dollar limit contributed into its plan.
These dollar limits are calendar year limitations. If you changed jobs mid-year, contributed earlier in the year to the plan of your former employer, and you can yet contribute in the remainder of the year to the plan of the new employer, you cannot defer in total into both plans more than the annual dollar limitations. You have the responsibility to monitor the total contributions you make into the two plans, not your new employer. You will incur penalties for violating these dollar limits!