You can choose the IPv4 and Subnet option and discover up to 65,536 IP addresses, which is the number of hosts with a class B network. Enter e.g. 10.0.0.0/16 to discover a class B network. Please have a look here for further information.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.