Hi,
Please refer to this documentation for the eDiscovery limits: https://learn.microsoft.com/en-us/microsoft-365/compliance/limits-for-content-search?view=o365-worldwide
The 1000 results you see are the limit of the preview.
In case you need to review more, you can try the method described here: https://learn.microsoft.com/en-us/exchange/policy-and-compliance/ediscovery/compliance-search?view=exchserver-2019#optional-step-2-verify-the-number-of-source-mailboxes-in-the-compliance-search
BR,
P