Look like you currently have "compo mode" setting. UI: In this (not default) mode, the % (on your picture, 5 for females) doesn't reset if you toggle (∞) the unit. Trainer: (on your picture) Will still work like you would expect: Train females, until further input from you. Example of how to use: Say you start the game and want to switch all production to slingers (either to defend a rush or to lead a assault on some woodline). Simply toggle-on slinger.
Once done (you have sufficient amount of slingers) you can toggle off and it will resume the composition training.
If you reset to the default mode in settings "spam mode" (We need to find better settings name lol):
Then you will either see only ∞ or the %, And the value will be forgotten when you toggle-on a unit.
Default settings, you will only see XX% or ∞ But more clicks if you like to change trainer inputs often.
Maximum batch size can be found in the trainer context menu along the reserves. Here 12. Can change value with mouse-wheel.