Hello Shocko,
I am sorry we don't have a feature can send alert when you are close to the limit. But there is a calculator you can use to have an idea about how much you will spend for your service, so that you can be aware of that when you are closed.
https://azure.microsoft.com/en-us/pricing/calculator/?service=cognitive-services
Please let me know if you have specific scenario for your requirement. We can help if we have more details.
Regards,
Yutong