Pricing what happen if we exceed the plan limit

I am reading through the pricing at https://manychat.com/pricing . but let say we chose the Business plan which come with 7500 active contracts. so i have these questions:-

1. what does active contract exactly means? whole chat? or per message, so a chat can consume let say 10 active contracts?
2. can we specify to stop replying if we exceed the limit? or to say if we go beyond 7500 active contract, to only answer questions up to extra 50 USD??

thanks