Senior cats (11+ years) typically need around 180–250 kcal per day, often less if they've slowed down. Following the 10% rule, that's just 18–25 kcal of treats daily — about 4–8 small treats.
For the calculator, choose "Low" activity for most senior cats. If your cat is overweight (very common in older cats), set the goal to "Lose" to get a reduced treat target.
Some very old cats actually lose weight and need more calories. If your senior cat is unintentionally losing weight, talk to your vet — it can signal kidney disease, hyperthyroidism, or dental pain.