For WordPress “multisite” websites, you only need 1 plan for us to manage your “main site”, to host the multisite, and perform all security and managed updates for the whole multisite network.
However if you would like us to manage, maintain, and perform changes on each individual sub-site (with separate themes etc) on your multisite network, you will need to create a bulk plan and add additional sites for each sub-site. Please feel free to get in touch to discuss whether or not your multisite sub-sites need separate plans.