How many lives can be gotten in 3-2 by kicking shells against other enemies? Surprisingly, 14.
This demonstration TAS is inspired by Kosmic's rare 1-Ups video:
• NEW ways to get 1-Ups in Super Mario ...
A completely different strategy is used, with a new technique, kicking the underground shell and catching it multiple times. It require precise inputs, and it allows the shell to travel a bit faster than Koopa's walking speed. The Koopa shell has to be kept to the end at least that far, otherwise one or two Goombas in the end may not show up (unless it's 2nd quest).
Although the solution may not be as straightforward as Kosmic's, I believe it's the only way to gain maximum lives.
I even attempted to kill a Koopa on the left with the shell. It's close, but sadly didn't work.