At the moment, Linked Helper does not have a filtering option for the number of connections, but there is a workaround: when Linked Helper visits the profile page, it collects information about the profile's number of first connections which is provided by LinkedIn:
So what we need to do is:
1. Collect and extract your 2nd or 3rd connections.
2. Open in MS Excel and filter those, who have more than 500 connections (network_info_connection_count column) and delete the others:
3. Upload edited CSV file to Invite 2nd & 3rd connections action.