With a quick look on both Core 2.0 and the W3C member submission for Resource Shapes, it looks like neither mention whether a client can expect zero-or-one or zero-or-many instanceShape links from a resource.
I would expect zero-or-many. As I can't see any problem with an instance having the properties from multiple shapes. This would help with the question about vendor-specific shapes raised on
OSLCCORE-25 - as then the resources can include a link to both the vendor-specific shape and the OSLC TC-minted-URI for the a shape as defined in the spec.
In v3.0 I think we should clarify this to state that a resource can specify zero or more instance shapes. (If we don't already & I missed it).