Search results returned by Microsoft Outlook are limited to 250 items by default, allowing for speedy searches.
The limit can be easily increased to either 1000 or unlimited results. The unlimited option requires editing a registry key (more advanced).
These settings have been tested on Outlook 2016, 2019 and 365. Other platforms are not supported.
To increase search results limit to 1000:
In Outlook, click the File menu -> Options
Select Search in the left column.
Locate and clear the selection for: “Improve search speed by limiting the number of results shown”
Additionally, if more results above 1000 are desired, a registry key edit can be used to increase the number of results to unlimited (more advanced option). Please note, this may cause Outlook to momentarily "freeze" while a search is ongoing.
Open the Registry Editor and modify/create the value:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Search\DisableServerAssistedSearch
REG_DWORD = 0x01