The shelter started with 12 / 2 = <<12/2=6>>6 cats.
The cats they took in increased the number to 12 + 6 = <<12+6=18>>18 cats.
Three cats were adopted, so they had 18 - 3 = <<18-3=15>>15 cats.
One cat had 5 kittens, which increased the number of cats to 15 + 5 = 20 cats.
The pet owner came to get his missing cat, so they now have 20 - 1 = <<20-1=19>>19 cats.
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