I don't know but very, very little. According to Dr. Google, milk chocolate has about 20mg of caffeine per 100g. A bag of malteasers is about 37g if I remember right, and I'd be surprised if it's even half chocolate. So 20g of milk chocolate would have about 4mg of caffeine. A cup of tea has about 50mg, for comparison.
Go ahead and eat all the malteasers your little heart desires.