tl;dr: The limit of 100 http callouts is the same for all transactions (be they synchronous or async).
The answer here resides in another piece of documentation: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_gov_limits.htm
Salesforce gives us a semi-visual representation of which limits apply to both sync and async transacttions, and which ones have different limits for sync vs async. As I'm waiting for sfdx to do a large push, I'll try to capture the formatting of that document here.
+----------------------------------+--------------------------+--------------------------+ | Description | Synchronous Limit | Asynchronous Limit | +----------------------------------+--------------------------+--------------------------+ | Total number of SOQL | | | queries issued | 100 | 200 | | Total number of | | records retrieved | by SOQL queries | 50,000 | | Total number of records | | retrieved by | | Database.getQueryLocator | 10,000 | | Total number of SOSL | | queries issued | 20 | | Total number of records | | retrieved by a single | | SOSL query | 2,000 | | Total number of DML | | statements issued | 150 | | Total number of records | | processed as a result | | of DML statements, | | Approval.process, | | or database.emptyRecycleBin | 10,000 | | Total stack depth for any | | Apex invocation that recursively | | fires triggers due to insert, | | update, or delete statements | 16 | | Total number of callouts | | (HTTP requests or Web services | | calls) in a transaction | 100 | | Maximum cumulative timeout | | for all callouts (HTTP | | requests or Web services calls) | | in a transaction | 120 seconds | | Maximum number of methods with | | the future annotation | | allowed per Apex invocation | 50 | | Maximum number of Apex jobs | | added to the queue with | | System.enqueueJob | 50 | | Total number of sendEmail | | methods allowed | 10 | | Total heap size | 6 MB | 12 MB | | Maximum CPU time on the | | | Salesforce servers | 10,000 milliseconds | 60,000 milliseconds | | | Maximum execution time | | for each Apex transaction | 10 minutes | | Maximum number of push | | notification method | | calls allowed per Apex | | transaction | 10 | | Maximum number of push | | notifications that can be | | sent in each push | | notification method call | 2,000 | +----------------------------------+--------------------------+--------------------------+