A HomeDrive subscription allows you to create an unlimited number of Vaults. Note, however, that there will be a technical limitation to the number of Vaults you can sync to your computer as drive letters are used. The restriction is based on the number of drives letters currently being used, but will be approximately 15.
If you require more Vaults than you have available drive letters then it is possible to suspend select Vaults from your Dashboard so they do not take up a drive letter. While a Vault is suspended it is not synced but it can be unsuspended at a later date so the sync process can proceed (when a drive letter is available or when you suspend a different Vault to make another drive letter available).