is there a mod (or ini change) to make more NPCs cast shadows on screen? what's the limit? thanks

There is no limit on how many NPCs cast shadows except the shadow cutoff distance in Skyrim. The INI setting for that is fShadowDistance in SkyrimPrefs.INI, but be warned that increasing that value will adversely affect the shadow quality.

Also be aware that you may take a performance hit as well. I've noticed that, whenever I have performance issues, turning off shadows generally takes care of (most) lag issues.

well i'm talking about when NPCs cast NO shadow when next to obvious light sources. like in Dragonreach, next to that huge for in front of Jarl. nobody was casting a shadow around that fire. down the steps in the area around the door they would cast. anyway, i found a mod included w/ the True ENB and Season of Skyrim mods. it's called Dynamic Shadows. that's now making shadows around the aforementioned fire, and i assume elsewhere. so i'm happy now lol

Which lights do and do not cast shadows are determined by the game's data files. As you discovered, there are mods that add more shadow-casting light sources, but there is a limit to how many can be in an area at once (four).