To avoid too much load, SharePoint Online implements
throttling which will suspend requests if exceeding the limits.
APIs have different costs depending on its complexity. These are tracked as
Resource Unit. An application is given a limited number of Resource Units depending on the number of licensed users in the tenant. These limits may change, so refer to the documentation.  Currently, 0-1k licensed users get 1,200 Resource Units per minute or 1,200,000 Resource Units per day.
- Keep the number of requests to a minimum.
- Distribute the requests across a period of time, rather than at once.
- Decorate your traffic with an AppID/AppTitle and User Agent string.