0.08mg per half cup serving, but may vary depending on water used. https://ods.od.nih.gov/factsheets/Fluoride-HealthProfessional/

10mg is the accepted daily upper limit for an adult. So 62.5 cups of oatmeal to reach that max safe limit. https://ods.od.nih.gov/factsheets/Fluoride-Consumer/

Acute poisoning symptoms may appear as low as 10-15 mg/kg, lethal dose might be 32-64 mg/kg. https://en.wikipedia.org/wiki/Fluoride_toxicity

So 200 to 400 cups of oatmeal per kg you weigh for a lethal dose. (32/0.08/2=200, 64/0.08/2=400)