If you create a vault on an external (lets say 1TB) hard drive, you can fill the vault up to the capacity of that drive (i.e. 1TB). If you want to use Cryptomator to encrypt a local storage device without synchronizing it to some slow (internet-connected) storage backend, I’d recommend other solutions, such as VeraCrypt.
Cryptomator is created with cloud synchronization in mind. VeraCrypt on the other hand is designed for hard disks. Without the need to encrypt every single file at its own (which is useful for cloud storage services) VeraCrypt doesn’t leak information like the number of files or their individual sizes.