How can I trick the Npc limit?
Long ago the maximum allowable NPC definitions per map was 36. Starting with the xocolatl release, the limit was increased to 100, and in zenzizenzic to 500. It's unlikely that you would hit this limit.
Never waste two NPC definitions on identical NPCs, because you can simply create one NPC and put more than one copy of it on the map.
You can have 300 NPC instances on each map. This means that you can have multiple copies of each of your 500 NPC definitions, but no more than 300 total.
The easiest way to work around this limit is to split your map in two. You can create a copy of the existing map in map editor, but populate different parts of each copy with NPCs.
If you have non-interactive NPCs on a map, you might be able to replace them with slices.