Hi,
As far as I know, there is no such upper limit. However, keep in mind that dealing with huge number of objects at once may impose a performance penalty due to the time needed to retrieve/transfer such amount of data. Also, it makes it harder for the users to navigate/search the data in the UI.
Usually, it would be better if you can organize your objects in object hierarchy; like for example a vCenter plug-in where several VMs can be grouped under a host, several host can be grouped under a cluster, and several clusters can be grouped under a datacenter.
BTW, what upper limit of objects do you want to support in your plug-in? Also, you can consider adding support for pagination (via implementing the interface IQuerySupport by your plug-in factory class) if the external system you collect data from provides suitable API to do it effectively.
