Do I have to add my w2 for an employer i was with for less than a month? I only made $400 from it, I don't know what the limit is.

Yes you have to add it.

Every 2020 W-2 that you received must be reported on your 2020 tax return, even if they are for small amounts.  Remember that each one of those W-2’s has your Social Security number on it, and that income was reported to the IRS by the employer.  You do not want to get in trouble with the IRS for under-reporting your income.

ALL of your W-2’s must be entered on the SAME tax return.  After you enter the first one, you click Add Another W-2.

W-2's come from your employer, and they have until February 1 to issue it.  Some employers allow you to import the W-2 through the software, but for security reasons you still need information from the actual W-2 to import it.  You might need to phone, drop in, email and/or snail mail an old employer to make sure they know where to send your W-2.

https://ttlc.intuit.com/questions/2561916-how-can-i-get-a-copy-of-my-w-2

https://ttlc.intuit.com/questions/1934390-how-do-i-import-my-W-2

https://ttlc.intuit.com/questions/2588751-why-can-t-i-import-my-w-2

https://ttlc.intuit.com/questions/2938482-how-do-i-take-a-picture-of-my-w-2-in-turbotax-online

https://ttlc.intuit.com/community/income/help/what-if-it-s-february-but-i-still-don-t-have-my-w-2/00/25998

All income must be reported. That’s the law. 

If you are required to file a return for other income you must report ALL your income no matter how small.  Even $1 of bank interest you don't get a 1099 for.  

Do I need to file a return?
https://ttlc.intuit.com/community/filing-and-paying-taxes/help/do-i-need-to-file-a-federal-return-this-year/00/25546