23 kills for ruins
19 kills for snowdin
22 kills for waterfall
44 kills for hotland and core
3 kills for barrier, judgement hall, and throne room
111 kills overall
these kills count trios, side characters, and others that can help you go past or do not count for the kill count. but if you choose the option to destroy the world then that means you can get a whopping 8.2 billion kills (when speaking in technical terms).