Zapier will send you an email once you've used 80% of your plan's task quota for the month, but perhaps you want more granular options. Using this zap template you can set a % or fixed number of tasks threshold trigger and send yourself an alert by email, SMS or any other means.
When setting up this zap, the trigger step give you two options. You can trigger when a certain % of your tasks have been used, or when you reach a fixed number of tasks used.
You can't set multiple thresholds in a single zap, so if you want more than one alert you'll need to set up multiple zaps.