What is the max deferral ?

A new Part III of Schedule SE, Self-Employment Tax allows self-employed individuals to compute a "maximum amount" of self-employment tax payments that can be deferred. 

Self-employed taxpayers are allowed to pay 50% of Social Security tax on net earnings from self-employment for the period March 27, 2020 through December 31, 2020.  The deferred amount will be paid 50% in 2021, and the rest in 2022.

The instructions for Line 18 are very vague, but also open-ended, allowing you to use any reasonable method:

Use any reasonable method to divide the net profit or (loss) you reported on line 3 between (a) January 1, 2020, through March 26, 2020, and (b) March 27, 2020, through December 31, 2020. A reasonable method will accurately reflect the time when income was earned or accrued and deductible expenses were paid or incurred, depending on your method of accounting. In most cases, a proportional division of the amount reported on line 3 based upon the number of days in each period will be considered reasonable.

Please see the IRS's 2020 Instructions for Schedule SE for more information.