I guess if there was a limit it would be 32, because 2048/64 is 32. But we know that every spawn doesn't have a name that is the max it can be. So I figured for 100 (just pulled this number out of my ass) and those get output to the listbox. However that limitation is on the window, not the actual ignore list. But honestly if you're ignoring more than 100 mobs in a single zone it might be worth looking at why you're ignoring that many and discuss your options for a potential to limit the creatures you can engage a different way.
The ignore list generally is meant to ignore mobs that have a positive faction with you, that you want to maintain a positive faction with (guards), mobs that cannot be injured (some merchants, various quest mobs, etc).
What it's not intended for is ignoring everything except the one creature you want to fight. For that you would want to utilize alert lists.
The plugins automatically ignore anything on alert list 1, so you could add what you want to kill to another alert list, and then in alert list 1 add the other noalert #, where number is the list you added the stuff you wanted to fight.
This may look like,
/alert add 2 race lion
/alert add 1 noalert 2
Then you would only attack a creature whom has a race of lion. This is because we ignore alert list 1, and by adding alert list 2 but saying noalert, it means ignore everything except what is on alert list 2.
docs.macroquest.org