You genuinely cannot max out your Charisma skill. A Tome of Leadership and Influence raises your Charisma by two and raises the cap on it by two. So, if you were to find a stack of Tomes of Leadership and Influence and they all retained their magic, you could keep reading and keep raising your skill. However, assuming we're just working with a 20 Charisma, giving you a +5, you would have six uses of Divine Sense per long rest, which, if we're being honest, is more than enough.