InfoPath's default connection that retrieves data from SharePoint will return the entire data set as a secondary data source. So, you cannot limit the number of items returned. Sadly, the best you can do is to set up a filter and limit the number of items displayed in the dropdown.
If you have a custom web service (write your own code or purchase a 3rd party tool, such as qRules) you'll be able to filter on the server and return only the items that match the user's entry. Here is a related tutorial from MSDN. Good luck!