No, it is not determined by your Adjusted Gross Income (AGI). It is determined by your earnings from self-employment.
The maximum contribution limit for 2020 is $57,000 ($56,000 for 2019), or 25% of your employees' eligible compensation. For your own contribution, the limit is 20% of your net earnings from self-employment, as determined under the SEP IRA rules*, whichever is less.
*The maximum compensation that can be taken into account for purposes of this calculation is $285,000 per participant in 2020