I think the max amount of liquid N that can be safely applied "2x2" in corn planting depends on a number of factors:

1) soil type/CEC

2) amount of other moisture available to the seed and seedling

3) true distance from the seed - what we call 2x2 is often 2x0 (same depth as seed) and in a turn or on a side slope can be 1x0 or less as the planter crabs.

Having said that, I have seen 15 or 20 gallons of 28% used "2x0" which would be 45-60 lb of N or 20-30% of total N for a midwest corn crop. On sandy or lighter soil in a dry year it would be less.  With a lot of turns or side slopes I would space the fertilizer another inch or more to the side where it will end up 3x0 or 4x0.   This works well if also applying some "popup" of 4 or 5 gal/a of 10-34-0 or equivalent in furrow.

A critical factor in any on-planter fertilizer, especially liquids, in both popup or 2x2 application, is to make sure your plumbing and distribution is set up to apply THE SAME FLOW TO EVERY ROW!

It is amazing how many planter plumbing setups end up applying more to the center rows than the outer rows or have rates that vary widely with forward speed changes.