How to figure maximum retirement plan contribution amount (compared to income)?

[cmorristax]

I know that my retirement account contributions cannot exceed my total income for the year. But what exactly is that income? Is there a line on my 1040 that I can use? I see total income (line 9), AGI (line 11), and taxable income (line 15). Is the calculation total income minus lines 2 through 7?

I have both self-employment and W-2 income this year. I am contributing to multiple retirement plans (self-employed Roth and Trad 401k, employer Roth and Trad 401k, and a Roth IRA). My total income will be close to my total retirement plan contributions this year. 

I know how to calculate self-employment income to figure my maximum self-employed retirement plan contributions, if that helps any.

[DanPaul02]

There is an upper limit to the amount you can contribute to retirement plans of all types. For those age 49 and under, the limit is $58,000 in 2021, up from $57,000 in 2020. For those 50 and older, the limit is $64,500, up from $63,500 in 2020.

[cmorristax]

I know the maximum limits by plan type. My question regards the other IRS rule that says total retirement plan contributions cannot exceed taxable compensation. My total income this year will be around $28,000, which means that's my limit. I will have more than that amount to contribute to my retirement plans, so I need to know the exact figure the IRS uses so that I don't over-contribute.