The iPad User Guide for iPadOS18 provides the published information concerning the new iPad Calculator App. Here is a direct link to the related section of the Guide:
Use the basic calculator on iPad - Apple Support
As is, there doesn't appear to be a configuration setting to limit the number of decimal places.
Apple does invite submission of feature requests via its Product Feedback portal. If you would like to see additional options within the Calculator App in future release of iPadOS, this would be the most appropriate channel through which to make a request:
Feedback - iPad - Apple