IRS has Retirement Savings Plan (RSP) limit of $19,500 for year 2020. What happens if I make more contributions than this limit? Do I need to pay additional tax now? (I am less than 50 years old, having only 1 full time job.)
Your employer will typically not allow more than the limit to go in there. Your contributions simply stop when reach the limit.
Note that this is not a good plan to try - if your contributions stop, the employer match stops too for the rest of the year, so you could lose out. Make sure to not reach the limit before the end of the year, so you can still get employer matching.