While you do have free access to the Standard Search API, you will have to apply for access to the Premium Search APIs and talk to an account manager about the Enterprise Search APIs.
You can find a breakdown of the different products, and details on the Standard API here:
https://developer.twitter.com/en/docs/tweets/search/overview
Apply for Premium access here:
https://developer.twitter.com/en/premium-apis
And Enterprise here:
https://developer.twitter.com/en/enterprise
Here are some details from our documentation on pulling estimates using our Enterprise level APIs:
Full-Archive Search provides a ‘counts’ endpoint that is used to generate a minutely, hourly, or daily time-series of matching Tweets. For use cases that benefit from knowing about data volumes, in addtion to the actual data, the Full-Archive Search ‘counts’ endpoint is the tool of choice. Note that the ‘counts’ endpoint is a measure of pre-compliant matched Tweets. Pre-compliant means the Tweet totals do not take into account deleted and protected Tweets. Data requests will not include deleted or private Tweets.
The Historical PowerTrack API provides an order of magnitude estimate for the number of Tweets a Job will match. These estimates are based on a sampling of the time period to be covered, and should be treated as a directionally accurate guide to the amount of data a historical Job will return. A Historical PowerTrack estimate will help answer whether a Job will match 100,000 or 1,000,000 Tweets. The goal is to provide reasonable expectations around the amount of data a request will return, and the Historical PowerTrack API should not be used as an estimate tool.
We do currently have a counts endpoint for the 30 day Premium API, but have yet to release a Full Archive Premium version.