Key Store has a collection of digital certificates, which are unique to a publisher, like your signature. They are used to verify that Apps come from a genuine source.
Package name + signature combo must be unique, i.e. one App can't have multiple official publishers:
- Device won't update same App package with different signature.
- Play store won't let you update same package file with different signature.
but a publisher can publish multiple apps:
- Device can have many Apps with different package names but same signature.
- Play Store can host many apps with different package but same signature.
Apart from that, each Play Store Account is given a unique Licence key, unlike signature, its bound to the Account. You can't use one Account's Licence Key for an app published from other account.
As far as Google policy is concerned, I don't think they limit 1 signature per developer account as of now.