OpenFlow uses ACL's to create flows, and static FDB entries if enabled. The limit of flows are directly related to what type of openflow/ACL flows you are creating, and where the ingress of the flow is.
Each switch in a stack has there own ACL slices so it could theoretically double the number of flows.
Example: X is the number max flows on a single switch
X flows from Slot 1 to 2, and X flows from slot 2 to 1
Hope this helps.