How do I find more monsters?

Solution 1:

Once you get close to completely clearing out an area, the random encounter rate nosedives dramatically. If you are serious about doing this run, it'll take some determination on your part.

Solution 2:

Zones only have so many monsters. Once you kill them all, they're gone. They are not infinite. When doing a genocide run, 'but no one came' is a sign you need to go on to the next area.

If you used cheats, you may have altered the monster count, your question is a little unclear about that. You shouldn't need two more monsters if the save point says 'Determination'.